This battle is obviously the easiest: you'll probably end it in one or two turns. Just throw immediately the combo L2 (Cloak and Dagger) -> L9 (Back Stab), and then finish the enemy with the Diamondback if needed.
This fight will be harder: Deadpool will use his correct attack sequence (L1, L2, L6, L9), so you'll have to save yourself with Phased. The best way to fight is to get Phased and use L6 (Sapper) on him, then strike with L9 on the following turn; hit him with L1 as long as you cannot become Phased, then restart with L2 -> L6 and so on. Anyway, watch your moves in order to be Phased when he uses his L2 (Bang Bang Bang!), or you'll be affected by Buff Blocker (Cannot gain buffs) and so you won't be able to get Phased.
In this battle one enemy uses Magic attacks, the other one has two Ethereal Stikes: staying Phased will then be a lot less useful. The best strategy is then Blinding your enemies with Spy's L1, keeping one of them Depowered with L6 and focus on the other. Try to keep Strange depowered (he'll often buff the party) and focus on Panther, striking it with L2 -> L9 when you're pretty sure to kill him (remember: Back Stab has Fatal Blow). Buff the Spy's health and defense, and if you want equip him with the Soothing Rest EISO, which allows you to gain health by Recharging.
This battle is a combination of the previous: Magik will often hit you with her L9 (Stepping Disk), which is an Ethereal Strike, then try to debuff you with her L2 (Unleash Hell). Stay anyway Phased as long as you can to avoid her other attacks, then attack her with a strategy similar to Challenge 2.
This battle features three robotic clones of the Spy, which will be very annoying to be fought due to their passive: they have a 20% chance to gain Phased before being attacked. Do not panic, since their damage output is relatively low: equip the Soothing Rest EISO and attack them as you want, planning well your actions in order to Back Stab them when they are not Phased. Keep your health high by Recharging a lot, since they'll usually try to multiple-Backstab you; it cannot be predicted when they'll do it, then you'll have to stay Phased as much as you can and hope in a bit of luck. Note: if they use Sapper on you (Depowering and removing your buffs), just Recharge and you'll still gain the Soothing Rest health bonus.
Cloaking EISO for Spy: Grants Right Behind You - Replaces Phased with Incorporeal, which works the same way but cannot be removed