Focus on 1 hero for your main as DPS to start. Use the auto equip button as you do maps until you get into C7. Flame aura and Lightening aura will be your best overall use but Cyborks you will need some sort of Tower in the lane , Cannons and Poison Dart towers are great for that. Aura can work but you need to learn placement and have enough Range on the them ( deadly strikes , range mod)
DD2 is about grinding out maps and upgrading gear and building out your defenses. Learning to know what works and what doesn't to counter enemies.
Collect Deadly strike , Destruction and Defense Ra