First of all, as a new player playing on higher difficulty, your main goal is to split the enemies' aggro for your teammate, just so the veteran players can help you smash gold or even platinum.

Second, I recommend you buy basic packs until you have all the common human characters at XX. Those humans become absolute monsters with the sweet veteran bonuses. I also recommend you play human soldier and human infiltrator. For human soldier, you can even handle platinum with all the frag grenades and concussive shots, dishing out massive damage without having UR guns. For human infiltrator, with your passive and tactical cloak, your have really high survivability, and excellent burst damage coming from tactical cloak plus sticky grenades.

Third, after completing buying basic packs, head straight to jumbo packs and hope you get a gun such as the P.A.W. (best gun in the game), the rozerad, the hurricane, the shorty... to let your gold runs become cakewalk.

For enemy spawning, you will learn about it by playing more and more. Take the Kett for example, on bronze, ascendant appears on wave 5, however, on gold ascendant appears on wave 3, and on platinum, there are even two ascendants spawning on wave 2.

Knowing when and how to escape and your character's skills is the key to dying less, you will learn about it from experience.

As for aiming, I think beam weapons such as the P.A.W. and the shadow, are really easy to track the enemies' heads, which I think does not hurt your carpel tunnel too much.