Ignore the first two thieves - the mimics will get them. Plant 2 mana crystals as fast as you can plant them, during the first wave. You can put these either right above the existing mana, or be daring and use them to try to protect that floor trap at the top of the board (going for the achievement).

After you have 4 mana crystals on the board, put something that blocks traffic with a shooter (tribal-ista, ballista, lightning tower, etc) behind it and/or a trap of some kind in front of it at the choke point that is in the center on the bottom. All traffic has to go through that one tile, so put your first defenses there.

At this point it's pretty simple. Just keep adding mana on the right, so that if any heroes get through your defenses they are slowed down enough that you can repair defenses behind them and bomb them, and so that you have lots of mana to spend. Wall blades are a good choice because thieves can't disable them, and whatever your favorite shooter is - be creative and have fun!