To get the key is simple, just go to the second building you see, go up to the second catwalk, and go right until you find it, its supposed to be spinning, with a red glow to it.
to hurt the baron making him lose more than 2000 health you must use a move that takes away all his focus then he restores it back to 100 focus but then the fun part....for 3 rounds he loses more than 2000 health good tip you must find out oh to fine the focus draining move choose the 2nd class