BerryJelly's Onslaught Deck (Onslaught 200+)
All credits go to BerryJelly:
Video with the info:
While it is impossible to show how to build every single lane and map with different mutator combinations, the best advice I can give is to really look into the mutator and counter it with the appropriate tower. e.g:
Slime/ramster/auras/earthshatter for armoured lanes, and remove bees entirely. Ramsters/heavy cannons/ballistas make quick work of frost lanes. Proton beam can be used very effectively for frost/cursi-kaze/boss lanes where there are no cyborks. Obelisks are great for distracting flyers from your precious towers or objectives. Poison dart tower and 1-2 slimes for Controlled Burn. Lightning strikes aura comes in handy when you have 20 spare mana, but can be used effectively to quickly dispatch zerkers/bosses too. If you are confident with your build, you may consider a riskier option of removing blockades to squeeze out more mana for dps towers.