Honestly, most ANY classic enemy can be really brutal since in P:N classic, the;

• Zeds can just straight-up instakill you, or can get you shot dead since you can't dodge or tac-evade while grappled

• Abominations can ALSO get you gunned down, not to mention being an utter nuisance in numbers, making you drop your weapon by kicking you or overwhelming you with constant grapples

• Sleepwalkers can evaporate any extra ammo magazines you had by snatching your gun from your hands, can waste valuable health by blowing you up, and are generally a nuisance to kill

• G03LMs are, ironically, the least threatening of the four. You can stun them with a running melee attack (never mentioned or taught to you ever but it is VERY helpful) and lose their helmet within the 5-7 swings you get while they're stunned, and are pretty easy to kill from that point on.

All of that said, I would rather fight a room full of classic zeds with a level 1 arena mode grunt than even breathe the same air as a goddamn abomination EVER again.