(If this guide is useful to you, then please rate it upwards.)

Let's face it, Brain In My Head is not a great game, by any stretch of the imagination. But it is cheap, so it has that in its favour - and it may provide some limited amusement for you, if that's what you're after.
Despite the game's failings, this guide has been written to help players get the achievements for it (except for the broken ones, of course). There are 25 achievements in this game, 20 of which are quite easy to get, and 5 which are broken (however, there is a way to get at least 1, or possibly 2, of those 5).

Most of these achievements for this game are self-explanatory and easy to get.

Mainly, this guide will help you to get the "Mr. Boss level 5" achievement, as well as the healing achievements.

The guide includes a section on the five "Broken achievements".
However, one of them can definitely be obtained, as I have the "heal 100 times" achievement - and I assume that the "heal 1000 times" achievement could be done by using the same time-wasting grinding strategy.
However, be aware that at least 3 of the achievements are broken and cannot be obtained, at least at this stage.
If you are a completionist, stay far away from this game, as you cannot get 100% of the achievements for it (unless they are fixed in a future update).

Levels
There are five levels in the game.
Each level has three waves of enemies.
After you have defeated the three waves, you then have to fight an enemy boss.

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There are several types of zombies, as well as the bosses.

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When you start the game for the first time, the levels will be greyed out.
Once you beat a level, it will be unlocked, and you can then play it whenever you want. For instance, if you have unlocked Level 5 and are killed by the Level 5 boss, you can leave the game and later on start it at Level 5, without having to beat levels 1 to 4 again.

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Be aware that Level 3 has exploding barrels - if you shoot one, and it is somewhat near to the others, there will be a chain reaction of exploding barrels, leading to your death.

Controls
Shoot weapon = Left mouse button
Alternative shooting = Right mouse button
Change weapon = Scroll mouse wheel
A = Move left
D = Move right
Spacebar = Jump
Left shift = Sprint
Q = Heal
E = Stage ability
R = Restart level

Annihilator
Kill 10 enemies
-- Easy enough, just kill 10 enemies.

Super Annihilator
Kill 100 enemies
-- Kill 100 enemies.
As you level up, you will get better weapons (use your mouse wheel to scroll through them) - however, be aware that some of the weapons have a blast radius, so you may damage your own health if you are not careful.

Mega Annihilator
Kill 1000 enemies
-- Kill 1000 enemies.

Okay, okay, this is just a beginning
Unlock 1st Level
-- Just play Level 1.

You really got a hold on me
Unlock 2nd level
-- Self-explanatory.

I'm moving on
Unlock 3rd level
-- Self-explanatory.

Almost there
Unlock 4th level
-- Self-explanatory.

No refunds, no more, no more
Unlock 5th level
-- Self-explanatory.

Mr. Boss level 1
Kill 1st Boss
-- To defeat all of the bosses, just keep far away from them, and keep on shooting them.
You want to keep your distance, so as to avoid their missiles.
Run around them in circles, or keep doing long runs from left to right and back again, which keeps their missile aiming off-balance.
Try to push them off the edge of the platform by the force of your bullets. Shoot at the bosses when they have their backs to the edge of the platform, i.e. use the direction of your weapon fire to push them towards the edge (if you shoot at them from the wrong angle, and your shooting pushes them towards the middle of the platform, then you are not helping yourself).

Mr. Boss level 2
Kill 2nd Boss
-- Self-explanatory.

Mr. Boss level 3
Kill 3rd Boss
-- Self-explanatory.

Mr. Boss level 4
Kill 4th Boss
-- Self-explanatory.

Mr. Boss level 5
Kill 5th Boss
-- See the next section of this guide about getting the 5th boss achievement.

Burn me, Baby
Start a burner on the second level
-- When you are on Level 2, use the E key to activate the Stage Ability.

Fall me, Baby
Make enemies fall on the first level
-- Shoot at the larger enemies to push them off the edge with the force of your bullets (the smaller enemies may just die straight away, rather than be pushed backwards by the bullets).
Try hanging around the edge of the map and let the hordes come to you.

Who said you can't fly?
Fall into the abyss on any level
-- Walk off the edge of a platform on any map.
Simple, just like falling off a log (or off a platform, as the case may be).

Sansara
Die once
-- Self-explanatory.

I know I can heal myself
Heal once
-- When you have taken damage, push the Q key to heal yourself.

This game makes me heal myself so much I can't expect this to be so please make a refund I can't take this anymore, okay?
Heal 10 times
-- When you have taken damage, push the Q key to heal yourself. Do it ten times (you may need to do it in the one life).

Meds, Meds, Sweet Meds
Find Meds on the level and use it!
-- I must admit I can't even remember seeing a med kit on a level, but I assume there must be one there that I accidentally ran over the top of and my character automatically picked it up.

Mr. Boss level 5
Kill 5th Boss
(At the end of Level 5)

Getting this achievement can be a bit involved, so it gets a whole section to itself.

The 5th boss is more difficult to get to than the other bosses.
You have to travel from platform to platform to get to him.
If you die, from damage taken, or from falling off an edge, you will have to start Level 5 all over again.

Here's how to do it:

You start the level on Platform 1.

There is a red transportation square that moves back and forth between Platform 1 and Platform 2 - jump on it and travel to Platform 2 (which has no enemies on it).
(Be careful not to fall off the edge of any of the platforms or red transportation squares.)



Jump onto the next red transportation square and travel from Platform 2 to Platform 3.
Kill all of the enemies on Platform 3.

Jump onto the next red transportation square and travel from Platform 3 to Platform 4.
Kill all of the enemies on Platform 4.

Jump onto the next red transportation square and travel (upwards) from Platform 4 to Platform 5.
Be very careful with this red transportation square - it can be very glitchy.
Only jump onto it when it is fully dark red - if you jump onto it when it is light red, expect to fall through it to your death, and have to start the level all over again.
If it appears to be light red, move yourself from left to right until it changes to dark red (this has worked for me).
Be very careful when you jump onto it, as I have had it take off without me (leaving me jumping into thin air; or waiting for the red transportation square to come back down, but it doesn't seem to come back down at all).
In fact, be prepared to have to do Level 5 up to this point again and again, in case that particular red transportation square fails (don't rage quit over it, just accept it as part of a broken game, and try again).





Kill all of the enemies on Platform 5.

There is a red transportation square which leads to Platform 6 and the final boss - don't jump onto it just yet.

There are some big enemies on their own platforms inbetween Platform 5 and Platform 6.
They will be shooting at you (they may even be doing so when you are fighting the hordes on Platform 5).
Hide behind the big pillars to avoid taking damage from their rockets; shoot at them to either kill them directly or to push them off their platforms by the force of your bullets.
When the big enemies are dead, and your health is at maximum strength (make sure of that before you tackle the last boss), jump onto the next red transportation square and travel from Platform 5 to Platform 6 to fight the final boss.



Be aware that when you are heading over to fight the boss, most of your weapons will disappear - you will only have one to fight him with. That's a bit sucky, but there it is.

Follow the tactics outlined above in this guide about fighting bosses.
Basically, keep shooting at the boss from a distance, whilst moving around a lot to avoid his rockets/bombs, and use the force of your bullets to push him off the edge of the platform.

The Level 5 boss seems to be harder than the previous bosses, so keep your distance (or you'll have to do Level 5 all over again).
Be aware that I have had to fight two bosses on the last platform at least once, so don't be surprised if that happens to you too - I'm pretty sure I have also faced just one boss on the last platform, so it may be some sort of glitch.

Doctor Robert
Heal 100 times (0.2% of players have this).
-- This achievement is basically broken; however, it can be obtained by careful grinding.
I tested it, by working on the theory that it could be obtained by healing 100 times in the one life (i.e. without dying), and I got the achievement.
To do it, just wipe out all of the enemies on one platform on Level 5, and then shoot at the ground near yourself with your rocket gun - if you shoot far enough away from yourself, you will take some damage without dying; then push the Q button to heal.
Wait for the Heal icon to reappear, and then repeat the process.
Do that 100 times, without dying, and you should get the achievement (as I did).

Exploit Master
Heal 1000 times
-- This achievement is broken (0.1% of players have this).
However, I suspect that it can be obtained by healing 1000 times in the one life (i.e. without dying), by following the process I outlined for the "Heal 100 times" achievement.
But I must admit that, so far, I haven't been able to face wasting so much time in healing myself 1000 times to prove or disprove the theory.

Crazy Annihilator
Kill 10000 enemies
-- This achievement is broken (0.1% of players have this).
Other players have concurred that this achievement doesn't work.
I suspect that the game wants you to kill 10,000 enemies in one life, but as there are not that many enemies in all of the levels, that just isn't going to happen.

Funeral
Die 10 times
-- This achievement is broken (0.1% of players have this).

I don't see how "0.1% of players have this", unless it's a glitch in the Steam statistics reporting, or it worked once but doesn't any more, or it's been hacked, or maybe even if it worked on the developer's account and he got the achievement.
The fact that this achievement is broken is easily tested by walking off the edge of a platform ten times, or being killed by exploding barrels ten times, or by being killed by enemies ten times.
If my theory is correct about each of the healing achievements needing to be done in one life, it may be that, to obtain this achievement, the game expects you to die ten times in one life, which is clearly impossible.
Sorry, but it's broken. Nothing to see here, move along.

Nirvana
Die 100 times
-- This achievement is broken (0.1% of players have this).
The cake is a lie; you can't play this game and get all of the achievements too (you can't have your cake and eat it too).
Just another broken achievement in this game.


Update 21 June 2017:
I believe that the three last-mentioned achievements are completely broken.
However, if that is the case, then I have to wonder why, when I wrote this guide (21 Dec. 2016) those achievements were noted on Steam as "0.1% of players have this" - but now, 6 months later (to the day), Steam says that "1.1% of players have this" (all of the 5 broken achievements are listed as 1.1%).
How is that possible?
If anyone has a work-around for those achievements, please tell us in the comments section below.