Best done with 2 controllers; 2 regular or a combination with a turbo controller/fightstick.
Once you pledge to a faction at the beginning of the game, you should take care of the Jump Ship trophy ASAP because once you switch, you will lose all progress you had.
If you play through the story, you should be able to get to level 50 by simple story progression, if not in the high 40s. To boost levels, and to make things quick, follow this guide:
1. Sign into your account with your preferred controller
2. Sign into a guest account with your 2nd controller
3. Edit your Kombat Kard to allow the most Faction XP (FXP) to be given WITHOUT the condition of "per Faction Kill." You'll see why below.
4. Go to Two Players -> Kustom Kombat
5. For your side, use the Danger modifier for both sides and the Infinite Super Meter for you in case you have Brutalities with pre-requisites but one is an X-ray. For the 2nd controller side, don't add any modifiers.
6. Pick a character you want to farm with. You can swap between characters if things get dull, but that means you'll have to perform step 5 again.
7. Pick someone on the 2nd controller who you wanna kill a bunch.
8. Choose your location. You can go with Random, but I recommend Lin Kuei Temple so you can use the fireball if your character has conditions for their brutality.
Once it starts, you should have 5 seconds to perform your brutality. So make sure you have a character doesn't require combos (e.g. Johnny Cage) because you only have 1 hit to do it.
When you perform the brutality, you should receive a nice boost. And because we're doing brutalities, the modifier for Faction Kills is null in void. Simply wash, rinse, and repeat until you get to level 50. Every so often, you'll get new rewards that will give you even more FXP per win, so it's a good idea to check every 5-10 levels.
Once you get to level 50, return to the home screen and switch your faction under Faction. You'll lose your large XP boosts, but you should have each faction completed in less than 2 hours a piece. Keep track of the factions you've completed as there's no indicator on which ones you've done already.