If you're going to trophies or achievements in Tiny Tina's Assault on Dragon Keep: A Wonderlands One-Shot Adventure, then you will want to reach level 35. The problem with this is that this is not as easy as you might expect, because the enemies in the game does not scale to your level.
None of the enemies in the game are above level 30, even at True Vault Hunter Mode. This means that most enemies give a very low amount of XP during the last few levels. So what's the best and fastest way to farm XP and levels to get to level 35 in Tiny Tina's Assault on Dragon Keep: A Wonderlands One-Shot Adventure?
The best level farming method in Tiny Tina's Assault on Dragon Keep: A Wonderlands One-Shot Adventure is to grind the four Ancient Dragons of Destruction (the raid boss version) if you can kill them in under 10 minutes. If not, the best place to farm XP is the final Badass round in the arena in Murderlin's Temple.
The grind to level 35 is a bit of a drag no matter which method you prefer, but it should only take a few hours if you get a decent strategy for completing the arena or the dragon raid bosses.
PS. this guide is meant for the standalone version of Tiny Tina's Assault on Dragon Keep: A Wonderlands One-Shot Adventure, and there are better places to farm levels if you play the DLC on the Borderlands 2 version of the game.