The enemies are not much of the problem with difficulty in the game, okay maybe the frequency of some enemies like the marksman in the jungle and the Abby in FER get quite annoying to quite difficult to deal with and the fact the Orc can act as a massive shield gets troublesome with the map layouts. The maps need more vertical movement since it's on a 2d plane and there's nowhere to go but into hordes of enemies. Expand the maps to allow more vertical movement so people can train and kite enemies around to either buy some time or separate enemies for those seemingly impossible missions. With a larger map size you can place more weapons around and/or have an ammo purchasing system like CoD Zombies for those high number rounds where you're basically guaranteeing to dump all of your ammo to be left with the starter pistol that anything past round 5 you're screwed with no chance of recovery.
Aside of the map layouts that need a bit of attention the weapons could use some balancing. for costing 3K the LMGs severely underperform the SMGs for a lower cost as well as having a reload time much too long to help with anything when the enemy can still push you backwards even when laying into their heads. In my opinion, reduce their ammo pool and increase their damage drastically especially with that flamethrower on the space level, picked that thing up, used it for 5 seconds and sold at as soon as that round ended. The B4A1 I believe you called it is fine as is except for it's long and unreliable reload speed.
Other than those gripes, this is done quite well.