These are merely guidelines. I've played with players with very little experience who were very good. I've played with players with twice my experience and nothing to show for it. In the end, experience is only an indicator of skill.

In this guide I use the term "core role". The core roles are: Money Collector, Crowd Controller, Tank Buster, Medic Picker, Single Target Damage Dealer, and Ammo Supplier.

Anyways, when people ask for my advice for what mvm difficulty level is right for them, this is what I initially tell them.

As the easiest difficulty, it's generally the place to get started in the game mode. Now you may be thinking to yourself, I have 2,000 hours of PvP experience, I don't need to start at normal mode. That mentality is why you were kicked for "no reason". That said, normal is incredibly easy, you will not learn much from it other than the core mechanics of MvM. I highly recommend moving up to intermediate once comfortable with the game mode.

Recommended Experience Before Playing:
None

This is a fairly easy difficulty level that is great for players who have played a bit and are ready to move past normal mode. Your time spent playing at this difficulty should include learning the basic role's of each class as well as gain an understanding that class comp is a thing. (Anything you learn about class comp at this difficulty, other than that it's something your team should consider, should most likely be disregarded. Class compositions that roll intermediate often struggle on higher difficulties.)

Recommended Experience Before Playing:
5+ Hours Of MvM
Have beaten all 4 normal difficulty missions at least once.

This tour provides a noticable difficulty bump from Oil Spill and is important for prepping you the higher difficulty missions. You should spend your time on this difficulty becoming proficient at every core role in MvM.

Recommended Experience Before Playing:
50+ Hours of MvM
Beaten every Oil Spill mission at least 5 times
Have beaten a mission from wave 1 as every class without refunding
5,000+ Robots Destroyed
25,000+ Money Collected

Now, before all the 2cidiots get mad at me for this section, please watch this: https://youtu.be/yKC6XmsLWVQ?t=129

Don't bother playing this one. It's rather easy and full of gimmicks. Additionally, the majority of players strictly enforce an innefficient method of completion. In general, playing this tour typically results in lowering skill.

Recommended Experience Before Playing:
lol, no.
Recommend: Not playing

If you do find yourself needing to play this one, the following sentence is essential to improvement while playing this tour: "No, you don't need a medic."

Now, I realize that this section probably enraged a few skilless individuals, so please remember our saying about commenting.

Finally hitting the higher level tours. This tour is a decent bit below expert difficulty, though is significantly more challenging than any of the other advanced tours. This tour is an essential stepping stone on your road to being an MvM expert. You should spend your time here becoming an expert at every core role.

Recommended Experience Before Playing:
250+ Hours of MvM
Beaten every Steel Trap mission at least 10 times
Profficiency with every core role
25,000+ Robots Destroyed
200,000+ Money Collected

ONLY PLAY THIS DIFFICULTY IF YOU ARE AN EXPERT. If you do not know the meaning of the word "EXPERT" then look it up. Most players who are going to actually be helpful at this difficulty won't take too kindly to someone who can't play <insert class here> or someone who "needs a medic". Think long and hard before you play this difficulty, because no one wants to teach a random in an EXPERT game.

Recommended Experience Before Playing:
500+ Hours of MvM
Beaten every Mecha Engine mission at least 50 times
Be an EXPERT at every core role
100,000+ Robots Destroyed
500,000+ Money Collected

Just randoming in to games is honestly not a very efficient means of improving at MvM. While it does provide many important learning experiences, many are better off finding a good player and asking for help. Most good players are not only willing, but happy to help newer players learn. Different players have different teaching styles, so if the first person you ask for help doesn't work for you, doesn't mean there isn't someone else who can help you. Most likely, after a short time you'll come to the realization that MvM, even expert mode, is incredibly easy; it only seems hard because the game mode is flooded with players choosing difficulties above their skill level. I wish you the best of luck in your endeavors, and hope that this guide leads to a happier and healthier MvM community.