@ Level 16:
Let all the yellow balls and one brown ball loose. They should fill the slot with 1 extra ball resting on top. Drop the plank and it will force the extra ball down which will slide the other plank over the gap. Let loose your final brown ball and win.
This game is easy if you play it right.
The point is not to go after his highly populated colonies, but try to get some of the smaller emptier ones. Wait until he empties out his big bubbles and then send a bunch of viruses to take them over. He will usually stock a bunch of bubbles in a few different colonies, which means that you will control most of the map and eventually outnumber him.
The biggest flaw in the AI is that it doesn't seem to differentiate between big bubbles and little bubbles. It will just go after what's easier to take, regardless of the strategic value.
Basically just upgrade your armor and spear, and build only spear men.
Playing the night elves and using the whirlers worked pretty well for me too. A whirler can take down just about anything. And if you send a few upgraded spear men behind them you'll plow right through the enemy.
The only way to kill the alien is to close the door, click the toilet, paper click the toilet seat, click flush the toilet, click the faucet on the tub, click the cupboard, click the item in the cupboard, click the outlet, click the sink faucet, drop hairdryer into the water, other cupboard door will fall open, click the gun and keep clicking the gun until the alien is dead.
I've found that Lightning Mages have the greatest effect on creeps. Poison Mages seem to have the least as the creeps are often resistant to it.
I've beaten all the levels by putting 2 lightning mages at the key points at the top of the map, and then I follow them with 2 fire mages and so on.
Dexers are pretty much worthless as they have such a small range. A mage does less damage, but if placed right will hit a creep twice as many times.