Well I was told to write a quick guide to boosting online so here goes nothing.
You need a team of 4 - 5 players nothing more than 5. The game should be set to 3 rounds (I’ll explain why later).You need to decide who is going to get the cash first. Once thats sorted go kill all the guards/security once this is done depending who is still alive, if you have a group of 5 two players will kill any other two players except the player who is collecting the cash ( if your only four players only one player can be the traitor). The player cash collecting will then kill the two traitors adding towards the Mr Popularity (kill 100 Traitors). He will the escape with all the cash working towards the Most Wanted achievement (Collect $150,000,000), the 50 to Won achievement (win 50 rounds of fragile alliance) & the *Mr. Play-It-Straight achievement.
The easiest scenario to do this on is Hot Coffee it has a reasonable amount of money on it and very quick to do. Naturally you can just arrange rounds where you want to boost other achievements like Mercenary (Complete one session in each F.A scenario) the DLC’s and the personal revenge achievements. Now by the time you have all other achievements (online) you will be left with Veteran achievement (200 sessions) now if you have been playing 3 round sessions you will get this at around 620 heist divide it by 3 (check the stat sheet) but if not it could take longer which is very boring to get quick sessions provided everyone in the group is on that stage you just start a round and throw your grenade and commit suicide so it would be like 2 mins a session. Well good luck hopes this helps.
*Mr. Play-It-Straight: There are many rumours about this achievement but here is a fact don’t escape as a Traitor play it straight as a merc 50 times