XP farm for novice account

Was just grinding for lexcorp gear and credits in survivor and found enough nth metal for a pro pack. Still needed Lobo and Superman for the challenge, and serendipitously an Injustice 2 Superman has come out of the pro pack.

Wanted to grind it up quick so it can help me with the expert challenge. Haven’t got any elite golds and only just beat prison superman in the campaign and was wondering where is best to grind xp.

Any advice?