The website www.seatguru.com has information and seating charts for Frontier and many other airlines, including advice as to which are the best and worst seats on each aircraft type.

Edited to add: Frontier doesn't appear to fly the A320, but the similar A318 and A319. Here's a quote from the airline's website...

We currently utilize a fleet of 60 aircraft including 49 132-passenger Airbus A319 jets and 11 114-passenger Airbus A318 jets.