I have all 5 of my Grenade Launchers up and running.
I find Medium as my preferred difficulty.
Hard is pretty annoying with how many of the Prototype enemies spawn.
I usually play on Adventure.
I can go AFK during Base Defense. Even a Tank has never breached my walls. NOTHING has ever breached my walls.
Once I double up my walls, I should be pretty invincible. ( Except to the bane of players hiding in houses. The FLAMETHROWER )