This guide is for people near the end of the game who are level 14 - 19 and looking for a good way to get extra XP.
First, I recommend that you complete the 'A Safe Return' mission, given to you by a green alien (in the Dark Forest close to the Wishing Well). Once completed the alien will give you the 'Ray Gun'
Now once you have received the Reindeer Earmuffs, go and speak to Santa. Once the fight starts, he will send out a 'Coal Monster' boss at you.
This boss will throw out infinite 'coal mines' and 'burning embers' which can be killed with one hit with the ray gun. They are all level 18 enemies, so will give out decent xp.
Keep killing these, making sure you are not caught in the explosion when shooting them, untill you are at the level you want. Keep in mind the quests after this one will also give you quite a lot of xp.
If the Coal Monster is close to death, but you still want more xp, let yourself die, come back and repeat the process.