The range of enemies is too high, especially considering how small the areas are.
There's hardly any room to kite the enemies in. Often I found the best solution was just to stand in the middle and mow them down hoping I wouldn't get overrun. If I tried to strafe around and kite the enemies I would often get overrun since enemies would come from all directions constantly, and if one of the bomb guys spawned it was almost always a death sentence.
I found the boss to be way easier compared to just the normal enemies. Since it was possible to run around and dodge its attacks compared to having to resort to standing still and just shooting.