If you talk to the explorer (Ned O'King) at the inn, he'll give you a clue as to how you might deal with the gelatinous cube. If you're a sorceror, you should by default already have everything you need to get past him.
There are a few different ways to do it, but I will just say that it IS possible to use attack spells outside of combat, if the situation calls for it.
Getting in through the front requires a disguise and some knowledge of the Morroi. Try talking to the folks over at Hermit Homestead in the grasslands.