Too Many Layers

I think most players probably see Sidekick and Suit Up and immediately think of X-23. Laura Kinney's most explosive strategy is built around her Honey Badger ally, and the ability to reliably have her in play throughout the game turbocharges X-23's damage output. Suit Up has the additional benefit of searching out Professor X to confuse the villain on turns where Laura has to flip to alter-ego to heal.

The Magneto pack introduced the Cyclops and Phoenix allies, both of whom are fantastic for X-23. Phoenix gives an extra use of Honey Badger and Cyclops makes all of those extra attacks more potent.

It's a pretty obvious and straightforward strategy, but it is a potent one. The main weakness to X-23 is always going to be that she takes a ton of damage and needs to go alter-ego more often than one would hope. Her obligation is also a killer, either forcing another flip or severely weakening your damage output. Still, very few heroes can match the damage a fully powered X-23 can crank out.

A few individual card comments:

Side-by-Side - I would generally be inclined to only two copies of a situational card like this, but it is so crazy powerful that three copies is perfectly fine. Claw Mastery plus Side-by-Side makes turns in the high-20s for damage pretty common.

Advanced Suit - With Suit Up being a key part of your strategy throughout the game, you need a proper number of Ally Upgrades to make sure you are always getting two cards. Having a 0-cost option like Advanced Suit or X-Force Recruit is a nice thing to have if your hand works out to have that last card leftover. Either one works, but the Suit has more upside.

Legion - X-23 does most of her own blocking, and Danger Room Training is a nice card to have in the deck to potentially put on Honey Badger. These are two good reasons to have an ally like Legion who can keep himself in play and generate crazy value.

It is important to note that once you have three of the four Ally Upgrades in play, you should probably not put the fourth out. You want it in the deck or discard for Suit Up.