Team ups are important and which ones you have qualified should dictate which enemy you select, this is my basic strategy for each one.
If I have Wolverine qualified I will usually go for Mystique, and ideally with a boom ready to go. Wolverine has to be the first hit to make her vulnerable and when he teams up you get double damage, so with a boom you can accomplish that before the ball even hits your flipper kicking out of the scoop and then will have both ramps ready to deal damage and a huge chunk of her health taken out.
Sabertooth is the easiest by a lot, so if I don't have any team ups I'll usually go for him first. However, ripping that right orbit should be your first order of business at the start of the game (to get to work on quests), and two shots there will give you the Domino team up which will drop it into the pops which also damage him. So I guess if she was the only team up I had I would go for him, she also awards super slings/pops so it's good to get that early on too.
Dazzler is probably the most useful for Juggernaut simply because it's usually the longest battle and she doubles the amount of time you have (Deadpools health). Your best bet with Juggernaut is to qualify the lil deadpool multiball so the light below it is green and the drops are down, then start the battle. This way you can lock a ball in there which activates ball save and you can safely bounce off the side targets. This is how you make him vulnerable, but it's very dangerous especially on Premium/LE models. You can knock the ball out and stack those modes, but lil deadpool mulitball isn't worth shit until you work up to super jackpots and even then I'm not sure it's really worth all the wood you have to chop in order to get there. I'll usually just leave it and focus on my shots until the lock times out and it will give it to me anyway.
Colossus doubles all scoring that takes place during a battle so it's great on any of them, just make sure you nail that finishing move. I've gotten nearly 100 mil just in the finishing shot with a quick battle, Colossus team up and a 2x playfield running.
Otherwise focus on weapons until you get to 75, then probably work towards wrapping up both quests for the second extra ball. With 6 multiball modes you can be in multiball almost constantly, especially on a Pro model. I got 930 mil with this strategy the other day, which is my best for .97
Oh and don’t forget to move an unlit BOOM letter to the inlanes every time you hit a ramp, that’s free button shots just for paying attention!