The only two places where Mxyztplk have beaten Spectre where in Worlds funnest and Emperor Joker.
Worlds funnest = Worst PIS I've seen (except for the celestial being greater than LT) Besides that Spectre was host-less.
Emperor Joker was also filled with PIS, Anti-Life Equation was stated to be a billion times more powerful than Mxyztplk, and the Spectre is superior to that, but then again he was host-less there as well.
Mxyztplk maybe is in the 5th tier (where Presence is in tier 0) or maybe even 6th or 7th.
When the Living Tribunal is in the 1st tier (TOAA being in tier 0).
Abraxas presence alone caused the universes to colapse all together.
When he got the Nullifier he could've destroyed them in an instant.
A incomplete Infinity Gauntlet is supreme to the Ultimate Nullifier ect.
There're maybe 10 (at least, that's what I counted to) in Marvel that can destroy all the universes.
Witht the weakest being: