Aimbots and wallhackers, that's about it afaik. the aimbots are easy, though the wallhackers may sometimes become problematic if they have some skill.
They're real fun to fight actually. It get's even better if you can get all the people in the room to join the fight against the hackers XD
get a riot shield/FMJdeagle/throwingknife/stungrenade + scavenger/stopping power/ commando
The trick against aimbots is to just tank their bullets until they gotta reload, then you just throw a knife in their face and scavenge. A little risky if they have an LMG, in that case you should walk towards them and use the shield to melee them.
The Deagle is your main weapon against wallhackers. when they run behind a cornerswitch to deagle and finish them off with a surprise in the face

it takes some practice, but the sense of achievement in making an aimbotter ragequit feels so good XD
NOTE:
if you want something that can be used against real player as well, get akimbo deagle instead of FMJ.
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Day[9] wrote:
"Tea is a lot like gold expansions - it helps you kill people."
- Day[9] Daily 337 -