If you are playing Nightfall co-op, there is an easier way to get this achievement.
Go into custom match, invite everyone you are playing with, and make sure JOIN IN PROGRESS is allowed for the game.
Set up your loadout however you prefer and start the match. Play till the end of the facility, but BEFORE you start the final boss encounter, have whoever is NOT host quit out to lobby, equip the Small Wallet relic and rejoin the session in progress (I recommend no more than 2 people quit out and do this, so the fight will be a bit easier). Those people will spawn in with a pistol and some skill points. Spend the skill points on whatever you have that's most useful, run back to the facility, grab the Venom-X, buy an LMG if possible, then go kill the boss.
**NOTE** You can drop money for other players in Extinction by aiming at the ground while holding LT, then pressing and holding A for about 5 seconds.
The achievement will pop when the game ends, and you just saved yourself a lot of trouble instead of trying to run through all of Nightfall with a relic.
And in case it was not obvious, the host will NOT be able to do this trick because the host is the host. You'll end the game lol.
This still worked for me as of the first patch that fixed Onslaught bugs. If you have any issues with this solution, please leave a comment with what's going on.