How are your Armoury upgrades looking? The hefty bosses are best countered by the big single target damage towers (the Colossus being the obvious example), so upping the damage and range on them will help. Then you can pile on the damage multipliers – remember Armoury upgrades for Shielders can also improve their % boost.
Something that might also help is getting your Colossi built well before you need them. Even though they’re really not good against small, numerous enemies, sniping an arachnid every 10 seconds will get them levelled up ready for the bosses, and levels make a big difference.
Carrier Mantises are also super slow, so slowing them even further can give your towers more time to work.
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