For running any GI Joe d20 (well, level based, so excluding M&M), here's a little shorthand conversion I came up with. (Basing this off the old 80's and early 90's GI Joe line, so for counting re-issues, I didn't use the modern ones)
To determine the character level of any character, go to their file card.
GI Joe:
Level = Enlisted Pay Grade OR Officer Pay Grade +3, +1 to level for each additional re-issue of the character with a later set (Snake Eyes wins big here, for example).
Cobra:
Leaders (Named/Unique Characters): Base of 6th level (or if equivalent rank is given, like Major Bludd, use it instead), +1 for each additional re-issue. +1 if they were the actual leader of all of COBRA at any point (Serpentor or Cobra Commander). If they are explicitly commanders of a specific type of minion (Tomax and Xamot, Night Creeper Leader, they are at least 2 levels above the level of their minions).
Minions (Ordinaries/NPC classes): Rated by how many promotions in type they have, normally.
Even Elite Minions (Elite versions of the Elite): Crimson Guard Immortals, Python Crimson Guard = 10th Level
Elite Minions (anything by definition elite, or promoted from Third Tier): Crimson Guard, Night Creepers = 8th Level
Third Tier Minions (Whatever are promoted up from Second Tier Minions) = Hydro Vipers, Strato Vipers = 6th Level
Second Tier Minion (Anything above the absolute bottom, or promoted from First Tier): Air Vipers, EELS, SAW Vipers, Motor Vipers, Neo-Vipers, Cobra Officers (old school) = 4rd Level
First Tier Minion (Entry level positions, or very low quality minions): Viper's, BAT's, Cobra Trooper's (very old school) = 2nd Level