PvE cards level up

A question to all you pve players… what’s the fastest way to level up the classes? I want a select few of the new weapon skins but my highest card is 14 with low level cards, (I only ever play pve at the start of an op to get my tour off to a start) I’ve been running the surge on insane, getting to the safe room and then going from safe room to end, restarting act, rinse repeat,
I got one from level 1 to 11 but it took a few hours (over several sittings) there must be a faster way to do this but if not, I guess those skins can stat where they are lol