Built a fun deck at the high levels. Plenty of crystal, a nice big stack of Portal Growth (I use ten), five Barbarians, 100+ souls (4-5x Reaper's Bounty) and a Repair. Go ahead and let the enemy walk through your portal, building up your gold and crystal stocks, buy Portal Growth at each opportunity, and as soon as you've used them all and can cast all 15x barbarians at once do so. Use Repair while the Barbs are enroute, and you'll get a nice big chunk of change. Typically I'm hitting 3k gold per match this way against all six opponents.
One tip would be to have the towers be smart enough to not fire a shot at a bug that's going to die from a shot already in the air. A lot of TD games don't get this right and it's always something I notice when they do.
This one seems hard but it's very very easy. Play most levels with one hand on the arrows and the other on the mouse above the 'pause' button. A lot of seconds pass between the end of one level and the beginning of another and if you pause while it transitions you don't 'lose' that time.
 "Cardian"){kind=icon}
 "Exterminator"){kind=icon}
 "This is the Only Level"){kind=icon}