Sacred Citadel

This is probably the most annoying trophy in that all 4 of your classes must be at least level 22 in order to perform all the different combo finishers. One trophy requires level 40 so one of your classes will already be done, but the other three could take awhile.

To level up naturally requires playing through all of Acts I and II, so it becomes a 0.5 playthrough for one class. So this route requires 1.5 playthroughs just to level up your classes.

Hopefully you have the Goat Lion level available (a bonus DLC level available in Act I). If so, it won't take very long. Contrary to the game levels, this bonus level is always unlocked so all level 1 classes can access it. Since one of your trophies requires you to play all levels in multiplayer you will have access (whether local or on-line) to a high level character. Use this high level character to complete the Goat Lion level for your level 1 class. You will reach level 22 right before the level's boss, or will end with level 24 if you complete the level. The entire Goat Lion level with Player 2 as a level 40 class only takes about 10 minutes. So this route should only take around 30 minutes.

Once all 4 classes are 22+, perform all of the combo moves systematically. Press then select "Combos" from the menu to see how to perform them. The trophy will pop after the last successful combo is performed.

There are 11 combos total for each class. I personally confirmed that you can achieve this trophy in town; you do not need to be fighting any enemies. It's been surmised that the Hunter's Pin Cushion and Mage's Stasis Blast require enemies but they do not. In fact, Stasis Blast was the last one I performed (in town) when the trophy popped for me.