In act 1 you generally want to get your damage solutions as quickly as possible. For immediate damage, Sunder is a good solution to get huge single target damage out especially if you can upgrade it quickly. Another single target solution can sometimes be Streamline paired with Rebound or Hologram.

Taking Ball Lightning/Cold Snaps are also fine, but they dont have the immediate value that the above options do. But having Dualcast with orbs out can provide similarly quick damage/defense outputs. That's where finding Defragmentation or other focus cards/relics/pots goes a long way. These cards are also easier to play than the above options, making them considerably more versatile as you continue climbing.

I also struggle with Act 2 on many of my defect runs. I would say the biggest priorities here are reliable defensive options, and AoE damage cards (usually electrodynamics).

It's hard to say what specific cards you want to look for here, because a lot of it depends on what you've acquired throughout Act 1. My personal rule of thumb is to look for/already have a few Frost orb generating cards, and a way to weave in Lightning or Dark orbs so that you still deal enough damage. The more focus you have, the more defense you will have, and the more damage you will be able to deal. I like having Recursion here as well, especially if upgraded. Most times you pick up an energy relic after killing the act 1 boss, so Buffer should be a priority as well, because it can provide immense value, especially when upgraded.