I'll keep this simple....
Stats:
-3L at a 4th Tier LS
-3.3 gpa (top 35%)
-Corporate Tax grade: A
-Federal Income Tax grade: A-
-Internship at a governmental agency dealing with property tax matters.
Is this enough to get into Northwestern's Tax LLM program? If not, what is the best way to increase my chances? Nothing against other schools, but I have a strong desire to stay in Chicago.

Probably not good enough to get in. Northwestern is not one of the top 3 program but they are still pretty selective given the small size of the program. Even not given this selectivity top 35% at a Tier 4 probably isnt good enough.