Just died in my most successful game.
After you get to a certain point you know that you are going to die eventually. The reason for this is that your health caps out at 1500 (ignoring sheilds), while enemy damage doesn't cap at all, so if you get into a position where you take damage from 4-5 of them at the same time due to falling into a group off a giant ledge or climbing up a rope ladder into them, they kill you instantly.
[activate bragging mode]
By the end each colored enemy had at least 10,000 health (and sometimes more then non-colored bosses) and dealt ~500 damage per attack. Bosses spawned 3 or 4 in 15 second intervals, after which there was usually a small break between them. Colored bosses had ~35k health. I think I was on level 3 for the fifth time.
I dealt ~5000 damage per second, and healed 500 HP per second (if attacking any of them at all), due to knockback once I started firing no non-boss was able to damage me at all (since even golems got pushed back to the ledge and permastunned within seconds). Frequently the golem bosses would slowly walk towards me and die before even touching me. Even though bosses spawned super fast, it wasn't really a issue since they usually died ~5 seconds after they spawned. Regular minions didn't get in the way of them dying either, since they got pushed back to the ledge, while the bosses are knockback immune.
[/deactivate bragging mode]
I ended up dying by jumping into a group of enemies that I was sure would attack slightly earlier and instantly taking massive damage.
Now, the reason for my success is quite clear: At the end of level two I got a item that allowed crits to reduce ability time for 1 second each.
With low crit and attack speed this usually ends up taking a second off your ability cooldowns. At slightly higher crit and attack speed levels this ends up reducing the cooldowns to about 1 second with 1 second variation (so 0-2 second cooldown for abilities). Once you get great crit and attack speed, you can permanantley use all your abilities. I could just hold down the V key, and use my suppressive fire forever. This gives you massive single target damage. If you get some anti-crowd items (electric ukelele, the explosive shot, the drill) you do massive damage. Add the stun item (as well as AOE effect items) and you permastun and perma push back all non bosses. After that its just scaling up damage by adding more weapons.
If I hadn't gotten that item I doubt I would have survived past level 3 the second time.