A Girl and Her Dog

Cosmo is a good dog. Protect her at all costs.

With Magik's ability to see the top card of her deck and to manipulate what that is with Limbo or Scrying, you will always be successful with Cosmo's ability. Keeping her health up. For added bonus, make sure Digging Deep or White Tiger are the cards on top of your deck to get their benefits when you discard the card for Cosmo.

Next, pump Cosmo up with Sidekick, Honorary X-Men, Danger Room Training, Inspired, Comms Implant, and Laser Blaster and you have a 4 THW, 4 ATK, 7 HP sidekick!

Using Side-by-Side and Game Time to keep readying Cosmo means you can potentially swing for 9 THW or ATK each turn and take no consequential damage. Having 7 HP also means you could take a hit from the enemy or a minion if desperately needed and still have Cosmo live and heal back up.

Nick Fury, White Tiger, Maria Hill, Ironheart, Kaluu are all added for more card draw to get the upgrades quicker.

Update: after putting this together, I saw a deck similar to it and wanted to give them credit as well: https://marvelcdb.com/decklist/view/37180/magik-cosmic-hellhound-1.0