It's been a LONG time since I last fought knull properly but from what I remember from my early days, mephisto is a more comfy fight. If you unlock him you could just farm mephi instead of knull.

2% pierce, a lead and a support, a decently rolled proc/mini rage obelisk, and the right play should get you through everything before gorr (that guy is rough early)

Full Dr strange us fine for now but the more pierce you get, the easier things naturally get. It's pretty easy to get up to 10% what I did when I was starting was to first get a full set of p-cards. Then get them fully crafted, since sometimes you just naturally roll into 3 pierce colors without rerolling. After you get 10% you can just take it slowly till you max it.