By consuming a consumeable item, you gain a small amount of XP. This amount can be raised with XP-bonuses, with which you gain up to multiple levels per consumed item.
This can help you gain levels easily without fighting. The only thing you need is time and HL.
Here's what you need:
First up, go to the Stage 2-5. There will be a lot of zombies, and one zombie on a red tile.
That one specific zombie all the way back is standing ontop of an +50% EXP Tile, and we want to get there to use it. Defeat all the other zombies, and then head towards the last zombie ontop the EXP Tile. Pick one character that has a lot of health and just lift the zombie up. The map won't end if atleast one enemy is alive. By lifting up that last zombie, the map will never end. The character that does the lifting gets damaged every turn, so be sure to heal him.
Take the character you want to level up and place him onto the +EXP Tile. Now you can start to consume the unopened soda via the item menu. I was at around Chapter 8 when I first started trying this, and I had all together +350% EXP via the Cheat Shop and the Tile, and the Triple EXP from the Assembly too. With this I got 2 level-ups per drink. Now you just have to drink your way up the levels until you don't want to continue anymore.