For anyone trying to get 100% achievements for this game the biggest hurdle can easily be the "unstoppable" achievement which requires you to survive 20 rounds on endless mode.
This is no small feat, you're trapped in a small enclosure, ammo and health supplies are much more finite here than in campaign, and it can also last a while but you could lose all progress after only a few slip ups.

When I was struggling to get this achievement I tried looking for guides and forms for advice but most of what I found boiled down to "Put the difficulty to very easy and turn the aim assist to max" and I'm not going to look down on anyone for using these methods (after all if the game devs cared about it then it was on them to put difficulty conditions on achievements), but for me personally the whole appeal of 100%ing a game is feeling like I overcame a challenge rather than feeling like I sidestepped a challenge. So I was able to get the achievement on normal difficulty and this guide will tell you some strategies I learnt along the way.

The first thing were going to do is play as the Priest, Journalist is okay on campaign, but on survival with our very limited health supplies we need the priests healing and invulnerability, otherwise we will be worn down by attrition.
When picking a map a common mistake people make is playing on "Garden". It seems so obviously the best right? Its based on the earliest part of the game which was the easiest, and it also has the highest average high score out of them. Well you see its also one of the tightest and most claustrophobic maps available, allowing little room for manoeuvring and making it easy to get surrounded. Secondly while only facing early game enemies may seem like an advantage at first, the limited roster results in just hordes of liquidators being spawned around you in the later rounds. And thirdly it has the most limited weapon selection for yourself.
The "Mountains" map is much better and improves on all 3 of these problems, it gives you the harpoon as an extra tool to deal threats, the enemy roster includes more act 2 enemies such as yellow cultists and crab men to dilute the liquidator hordes without including any of the really dangerous late game enemies such as observers or abyssal hunters, and most importantly, its a large open circular arena with plenty of room to move no matter how many enemies it spawns in.

The upgrades you choose could make or break your attempt, here's the ones I chose that got me to round 20 on normal.
When playing on Mountains before you even start you have 3 upgrade points, you will want to spend all 3 on getting more xp from combat, this investment will quickly pay off.
Then upgrade the cross 3 times, the only health you will be getting in this challenge is either from the small enemy drops or your cross, so you want your cross to be as strong as possible.
Then for weapons, you can choose these in the order you want based on which ones you use the most but here's the routes I chose:
Revolver: I upgraded this into the Dual Revolvers, while I prefer the Noller in campaign it does less damage per bullet and we need to be more conscious of our ammo economy here, plus it opens you up to a health upgrade, don't underestimate these as they can really add up and save your life during a scrap.
Shotgun: I upgraded this into the Power Shotgun, as fun as the Abyssal Shotgun can be this arena is far too open to make any use of its ricochet effect, also another health upgrade.
Rusty Vito (Tommy-Gun): I upgraded this into the Rustier Vito, I find this more ammo efficient than the Goules (Beam Gun) and it costs 1 less upgrade point.
MP28 I upgraded this into the Nasstod (Grenade Launcher), this saves on handgun ammo, and is also very helpful with the larger groups of enemies you will face in the later rounds.
Harpoon: This is the weapon I'm the most indecisive about, I personally upgraded it into the Whale's Bane because its good against groups and harder to miss with its spread, but if you're confident in your aim you could easily justify getting the health upgrade instead, so I leave that to your decision.
Knife: I also recommend upgrading this into the Mutated Throwing Knife, but not until the game starts spawning in crab men. This weapon is frankly dreadful but it has infinite ammo so can be used to save ammo for your better weapons.
Other Upgrades:
Once you get to round 5 I also recommend you get 2 ammo upgrades to keep you a bit safer from running out.
Every 5 upgrades I recommend you put one point into your invulnerability book, once you have all 3 also get the health upgrade that becomes available.
And finally once you have everything else consider upgrading the lantern if only to get the health upgrade that unlocks.

In the early game I recommend trying to save ammo by taking out ranged enemies first and using your knife on wretches and gobblers (if you're not confident about getting into melee without getting hurt you can always just get the throwing knife upgrade early). However this is only worth focusing on in the first few rounds as it will soon get pretty hectic, especially at the start of each round where all the enemies spawn in at once. But the good news is that the higher enemy counts means more uses of your powers. So to counter the rush at the start of each round I recommend using the invulnerability book just as they spawn in, this will buy you a few seconds to focus fully on culling as many of their numbers as you can before it runs out.

You will naturally want to prioritise killing squad leaders and liquidators if you can but don't worry about it too much, the only important rule for target priority is to not waste time/ammo shooting at the crab men, they're big old bullet sponges and practically harmless too so leave them for last.

For the initial rush at the start of each round you will want to use the Power Shotgun, Grenade Launcher or Harpoon to deal with enemies quickly, but once the initial rush wears off I recommend switching to the Vito or Revolvers to whittle down their numbers further. And finally once only the crab men remain use the throwing knife on them to save ammo. Repeat this order of weapon use for each round.

One piece of general advice is that I believe a common mistake people make when getting into boomer shooters is to prioritise attacking over dodging, but not taking damage should be your top priority at all times. Remember you will need to defeat many hundreds of enemies again and again, but they only need to defeat you once. So don't risk your health in order to damage the enemy, as long as you keep dodging their attacks opportunities to attack them back will present themselves safely.

Aside form all that the only other piece of advice I can give is don't give up. The sense of frustration when you die pales in comparison to the sense of smug satisfaction when you succeed.