Well, I see what you mean about level disparity, especially Gandalf... but in his case, you have to remember that Gandalf is literally the avatar of a god, so you can't really hold him to mortal standards.
Avatars aside, in the Fellowship of the Ring, you have starting adventurers (the hobbits) alongside high-leveled party members (everybody else). While a couple of them have some minor magic (Aragorn, the party's official ranger, and Legolas, who's either a ranger or fighter, depending on who you ask), they're mostly nonmagical. The hobbits are mostly commoners (use rogue stats, if you don't have Folkways available), other than Frodo, probably all level 1, with the debatable exception of Merry (who might be level 2, owing to his position in the community).
Boromir is a human fighter, probably somewhere around level 5-6.
Gimli's a stereotypical dwarf fighter. In fact, you could make a strong argument that Gimli is what created the stereotype in the first place, including the distrust of magic that no other dwarf of Middle Earth displays so openly. I'd place his level somewhere around 8-10, depending on how much action you think he's seen (conflicting clues in all three novels).
Legolas is an elf, either a fighter or a ranger (but not a Ranger!), and based on certain statements he makes in The Two Towers, he's at least 1200 years old. By HARP standards, this means his level could be anything up to low triple digits, but let's conservatively say he's level 20.
Aragorn is something like 90 years old in the novels, and thereafter lives to be more than twice that. He's obviously a ranger. He's also a Ranger (i.e.: Dunedain). This means, ultimately, he's a Numenorean, a High Man, and thus a human with elvish blood which still flows strongly through his veins. In a HARP game, I would use greater elf blood talent to represent this, including the option that gives the bonus to stat mods and increases the lifespan. This helps explain why he's so high-spec. I would say he's probably level 12+ as a ranger.
So yeah, that's some pretty big differences in ability levels there. But then again, that's what happens when you have reluctant heroes mixed in with career badasses.
And just so we're clear, 120 magic resist at level 6 is pretty high, but not unbeatable, nor too far outside the realm of what you might reasonably run into. Resistances for max ranks at level 6 gives 71 points, racial bonuses could be anywhere from 0 to 20, and that's before you add in stats or talents or equipment. 120 is totally doable.
Plus, keep in mind that anyone tossing spells at such a character gets to add their own spell's bonuses to the mix, so a villain of a similar level could easily have similar numbers, especially if it's the bad guy's favorite spell. On top of that, spells are skills too, and thus it gets an open-ended roll. Hit that sweet spot at the top of the scale, and suddenly that high magic resistance is all but meaningless. Trust me, I've been on the receiving end of that more than once.