As someone who tackled a very similiar situation, being a hyper intelligent super A.I got threw in a fantasy world. I got stuck between exploring the A.I's previous world and the fantasy world, things blow out of control very fast the more you think deep about it. How the A.I came to be, it's history, how it affected the world, what are it's thoughts about humanity, what does it think about the fantasy world, what are it's ambitions. Then the fantasy world, you have to make up a history, the magic system, the factions, what is going on when the A.I came. A whole mess that kept me up at night until I decided to scrap it all and developing a whole new concept of post-apocalypse world.
The advices I can give you from my failure.
First make the MC a normal soldier, ground infantry and or a specialize unit that isn't flight. Or a Space Marine level soldier. Or anything that doesn't contain flight.
Second, nanites. Nanite is your best friend, why need healing magic when you have thousands of tiny machines in your body repairing it?
Third, the A.I companion should be biased since, logically, it would hold it's homeworld on a higher pedestal regardless of what MC thinks. Also, the A.I gives an excuse for info dumping about the MC's world, comparing the life between two different world.
Fourth, no higher dimensional eldritch entity. Trust me, your brain will thank you for not dragging them into your novel.
Fifth, for energy resource problem. The robotic suit could run on bio energy as the secondary source once the primary one eventually run out, so as long as MC still lives, the suit will not go offline albeit at half power. This also solve to security problem, as the suit would only fuction when the MC use it via bio signature. Or going my route, a near unlimited power source found in the far future.
Sixth, the MC would need to disguise themselves as a resident, so the best excuse would be the robot armor is a symbotic golem. And vice versa, the residents of the fantasy world would think roughly the same.
Seventh, nanites, again. When the suit is eventually damaged, you can once again use nanites to repair the suit.
Eighth, Runes or magical carving could be the way to upgrade the suit. This give leeway to friction between MC and the A.I, as in the MC want to make the suit stronger by carving runic symbols into the suit itself, much to the horror of the A.I. Though this would contradict with Seventh, the best way to solve it is the nanites only repair major damage and ignore minor ones like scratches and the like. And battle scars are awesome.
Ninth, since MC won't know much about the fantasy world, the typical and best excuse is amnesia. Unless MC wants to be open about being from another world.
Tenth, the most important is how did MC came to the fantasy world. The best way is to use teleportation or warp gate malfuction if you don't want to touch on the subject of gods. For a sci-fi setting, it's logical for the to be warp portals and teleportation devices.
Sorry for the lengthly reply, I got really excited when someone wants to write the genre I adore but seen relatively little of.
And the advices I gave should only be use as advices, do what you want, write what you want to write, it's the freedom that writing gives make it special. <3