There are 2 Painkillers on the counter when you first go down the stairs if you need them. Do this with haste because speed is of the essence. Next the guy who comes from the kitchen, then the last two in the back room.
Each game of Gang Wars will cycle through 5 rounds of different game modes where each team will have to complete different objectives against the other team (see Full Monty for an overview of each game mode). Gang Wars Mode: Gang Wars is by far the best game mode to earn a lot of XP in a short amount of time.You can view these in the Action list of online Grinds. I highly suggest getting in the habit of looting as not only will you earn XP from doing so but there are also numerous Grinds related to looting.
We suggest switching it up by playing both online multiplayer and offline arcade mode to level up. Although the offline method is quicker, it is much more of a grind. Multiplayer has died down a lot in recent times so you may spend a considerable amount of time just trying to find a lobby/game with other players. The trophy will take a lot of time to obtain and largely depends on skill and patience. You can rank up and earn XP both online and offline.
You will require 619,500 XP in order to reach Rank 50.