Haven't really found a way to stop the relentless enemies at higher levels.
Against everything else, a lot of lightening towers seems to be a good strategy. If you have enough of them, you can effectively pin down enemies, with them barely moving. Have had three different sets of enemies on the map before because lightening towers were slowing them down so effectively.
If anyone has any strats for the relentless enemies, I think using a combination of lightening towers and that would be unbeatable.
There is a glitch toward the end-game. Basically, if you get too much money, the game won't actually give it to you.
So like, I scored something like 650,000,000 in money, but the game gave me like 20,000,000. Kinda sucks.
This should be fixed now.