Whenever you fight a boss, your chance for special items dropping is increased by having rested xp available when the boss dies.
The easiest and quickest way to do this is to switch chars for a second so your char gains like 0.001% rested xp, more than enough to get that bonus.
You can also Bodyslide to base and back but the loading screens take a little while.
Some bosses like Magneto and Doom have rested xp "built into them" by jumping through the portal into the boss room your xp bar turns blue but others like Mandarin do not.
This is easily possible for all bosses although Modok can be a bit of a pain with his adds.
Remember, this isn't about getting more xp from bosses but to increase your chances of special drops from the boss.