Rabids are a pain in the ass only when on foot, in a vehicle they're easy, especially with the Trespasser sniper rifle equipped that goes through armor like it's not there, the vehicle's guns get that property when it's equipped. .
What's the purpose in owning a Grenade Launcher? The sheer inability to continue life without it ! ! !
Skeletal Mages are probably the #1 most annoying enemy in the game. At least with stalkers, you can still hit them while they're cloaked and elemental effects continue. Mages can't be hit while cloaked, and as soon as they are cloaked, any DoTs or slag wears off immediately.
Heavy Nomads can also be difficult, especially if they happen to have an elemental launcher. Blech.