At the moment we have 2 immediate Actions in the game. Both come from Rain. Infernal Wrath is straight damage so it's fine to apply retroactively. Even if it kills a creature, they will have already attacked so no worries. Retcon is very minor.
Aegis of Shielding reduces damage. It only marks one creature and it's pretty easy for me to apply dmg reduction as part of the monster round.
This will become more complicated as Rain levels up. I know as my main PC is a Hybrid Shielding Sword mage.
Er, yeah, somewhat.
Rain also has this:
Guardian’s Counter (Encounter Immediate Interrupt Close burst 2 ✦ Martial)
Trigger: An ally within 2 squares of you is attacked and you are not included in the attack.
Effect: You and the ally shift 2 squares as a free action, swapping positions. You become the target of the triggering attack in place of the ally. After the attack is resolved, you can make a basic attack against the attacker.
I didn't throw this out as an option in this one (as I type), because I didn't really fancy sitting under a net (she's all melee at the moment, you see) and Rain was attracting plenty of attention anyway. I did leave it out as an option for players to pick in the skill challenge at one point and will doubtless do it again.
I've not used this yet, which brings the Aegis back into play:
Sword of Sigils (Encounter Standard Close burst 1 ✦ Arcane, Force, Weapon)
Target: Each enemy in burst.
Attack: +7 vs. AC.
Hit: 1d8+4 damage and the target is marked until the end of your next turn. Until the turn ends, if the target makes an attack that does not include you as a target, it takes 4 Force damage after the attack is resolved.
Aegis of Shielding: If a target marked by this power hits a creature within 10 squares of you with an attack that does not include you as a target, you can use an Immediate Interrupt to reduce the damage dealt by that attack to any single creature by 3.
Booming Blade lets folks potentially kill themselves if they run after being hit, too.