Strange isn't immortal?
He's more immortal than Loki, Loki is immortal by way of not aging, Strange is immortal by way of Eternity making him unable to truely ever die until he feels it's time.
They've faught twice that I know of, once before Strange was a sorcerer, yet he still stalemated Loki for a bit.
Second time Loki had to steal all of Strange's artifacts to have a shot at winning.
The problem is, Loki is limited to his internal magic, Strange is only limited to whatever god's power he wants to invoke.
With prep, Loki is as good as dead.
Raw mystical power doesn't mean too much, what really matters is how you apply the power. There is a reason why Loki has been around 50,000 years and his power hasn't grown since his first 50.
Strange was able to pass him by in power after being around for only 30 years, he's just better at it, he was chosen millenias ago to be the greatest.
Now since Strange has accumulated a few more thousand years experience, his odds of winning are pretty high.
I still think Loki can manage 3/10 possibly 4/10.
You fail to realise Loki has various other abilitys besides magic, Such as his shapeshifting and transformation. Also his ability to inbune others with power (he created Absorbing man, who has matched Thor in battle)
Also, his power has inproved considerably since his fight with the Avengers way back when. He didn't have rune magic back then.
Strange still takes it proboly, but Loki is AT LEAST 4/5, possibly 4.99999/5.00001
And shapeshifting/transformation will get him....where? He doesn't have the ability to give others powers naturally, he uses asgardian magic, like with Absorbing man, he used magic herbs.
Loki has Rune magic now? Strange has had it before too, in fact, he had a rune stone he used to give Thor amazing powers.