Left 4 Dead 2

This steps works for severals bugged or hard achievements, not only for "Beat the rush".

A quick guide for those poor souls whom, after the LAST STAND update, could'nt unlocked some achievements as "Beat the rush". I will be your guide, let me introduce you how to do it (legally) in a few simple steps.

Simply start any mutation on Expert and then kick bots so they won't rescue you and then die so the achievement progresses. Do the same with 5 mutations and you should get the achievement by now.

https://steamcommunity.com/sharedfiles/filedetails/?id=121144623

Simply use this map and type "map charger_hs_training" in console (without the quotations).
Do not type "sv_cheats 1" as they will entirely disable the achievements.
Kick all the bots and pick up a melee weapon. Keep in mind this will take a bit of tries, so wait till you unlock the achievement.

Easiest done in Mutations mode. Select "Gib Fest" mutation for M60 with unlimited ammo, and no reload required. Now simply play any chapter on any difficulty and just hold LMB and kill zombies until the achievement pops.

To get this achievement easily, start a campaign on Expert difficulty. For that I would recommend Dead Center, and once you're in the campaign, kill all the bots and change the difficulty to "Easy". Then proceed until you see the 3 survivors highlighted behind a door. Open the door and the achievement should pop up.
Hope this helps!