Surtur wanted to ignite the Twilight Sword with the Eternal Flame in Asgard, so he stormed the city and, right when he thought he'd gotten to the Flame, it turned out to be an illusion Loki had made. After that, Loki mocks Surtur for a while, then blasts him in the eyes, then creates a bunch of doppelgangers of himself to confuse Surtur. Basically he was just occupying Surtur's attention for a few seconds, until this old soldier guy on Earth closed the Cask of Ancient Winters that was keeping Odin imprisoned in ice. Loki never really fights Surtur outright, he sticks to guerilla tactics and uses his maneuverability to his advantage. The scene makes sense; it's really just Surtur being his usual dumb, brutish self while Loki fights smart and stays one step ahead of him. It's clear that Loki has absolutely no chance of ever actually beating Surtur, though.
I'm not sure how he'd stack up to Dr. Strange now, but back when Dr. Strange was just "master of the mystic arts" rather than Sorcerer Supreme, Loki owned his ass with very, very little effort. Strange has gotten much better since then, though. He might be beyond Loki's power now, or they might be about even. I doubt Strange is weaker than Loki anymore, though.
It's been stated in canon that he's third in the ranks of Asgard's sorcerers, after Odin and Karnilla. Not too shabby given that Odin wields the Odinpower, which is basically like Asgard's version of the Power Cosmic--it's the power of the entire realm funneled into one person--and Karnilla is queen of the Norns, who are themselves very powerful magic beings. Loki's just a descendant of giants who trained his ass off in magic.
Tiwaz's power level is pretty much unknown considering we haven't really seen him do anything. He just tended to Thor once after Thor got caught in an avalanche.
I'd forgotten about the Enchanters. Enrakt was powerful enough to possess Mjolnir, so yeah, they're definitely up there.
