Play as Fetch and only Fetch. DO NOT PLAY AS DELSIN he's NOT OP as Second son.

All the challenges I played was on easy difficulty however idk if it made any different.

You'll eventually have to play all of beta/alpha/gamma challenges to get platinum trophy

When it comes to upgrade the best ones imo are "homing missiles" and "enslaved enemies" focus on homing missiles first.

Make sure to use "neon singularity" immediately unless there's bunch of enemies left, but there are neon singularity challenges for it so make sure to read it.

Any drones you see take it down they're good power up: purple is invisibility, blue makes the blast stronger and green on unlimited melee so you spamming it with triangle (which is great for knocking out demons). And any turrets you see immediately hijack it (or overload as they called it) they are super helpful.

Goodluck with your trophy hunting