So I just finished the game and have 100%, but am only level 19 with lots more XP to go. My quest now is to meet Sandy at the gate, if I do that will it end the game, and never allow me to get the level 20 trophy? 
 
And if not, does anyone have good grinding places, since I'm out of quests :=\

The game "ends" when you enter the gate, but you can go back to the main menu and continue your game.  The game saves right after you defeat Lord Von Prong.
 
And for grinding places... I switched between the enchanted forest (unicorns are the highest level enemies), and turtle lake (a lot of turtles in a very compact area).

I switched between unicorns and orque-village/prongcastle orques - i think especially the castle-orks give most exp in the game

" I switched between unicorns and orque-village/prongcastle orques - i think especially the castle-orks give most exp in the game "