Demonic Spiders: Energizer towers. They drain any ability you deploy quite rapidly, then use the energy to resurrect every single tower in range. And their range almost covers the screen. So basically, you can't use any abilities around them. Thankfully, Airstrikes work well on them.
To add insult to injury, the game tells you to stay out of range (not really possible) or take cover behind buildings, then puts you in a level where there is almost no cover.
Goddamned Bats: Hack towers. They can't actually deal damage, but they make your own units turn on you if you get too close, which makes it hard to cover them with your abilities.
Just to put things in perspective, imagine a person with a British accent using terms such as "Butts", "Hell yeah!", "Someone order takeout?" "Jabronis" and, "This ain't no beach vacation!", along with phrases that just don't make any sense, such as "Those who hesitate capitulate".