Elminster is an old-ass wizard from Forgotten Realms, oft-cited as one of the main reasons why that setting sucks shit. He's powerful enough to solve any problem that would challenge PCs, and no real reason not to do so which isn't utterly contrived. He's an Urza figure without the apathy or plane-ruining fuckups. Oh, and he's also Ed Greenwood's self-insert, the original Mary Sue of Faerûn far before Drizzt was a thing.
He's slept with more women than Greenwood ever will, including Mystra, the Goddess of Magic. Just because. Dude's left so many single moms and fathered so many bastards that one of the novels centers around one of his abandoned daughters robbing his home, and he lures her in to be used as a pawn in fighting a conspiracy of wizards. What a dick.
He used to be part of an article series in Dragon Magazine called The Wizards Three, where he would meet up with two other wizards to swap tales, gorge on Earthly junk food, and share custom spells, which were reprinted at the article's end for use in your D&D games.
Not to mention that he is an ally of the Harpers. You know, the organization that is (in part, the other being the gods themselves) responsible for Forgotten Realms to be locked permanently in a state of medieval stasis (due to them taking away all the various technologies made by people even if those technologies are beneficial to everyone). Thus he, along with them, is the reason why nothing changed in the setting.
Despite the Mary Sue levels he generates, Elminster didn't always used to be some OP Wizard like he is these days. Back in the past, when D&D was new and Forgotten Realms was slowly shaping itself, Elminster used to be a max-level Wizard with additional abilities other Wizards didn't have access to and some nice gear. His only shtick was appearing as a ex-machina to the players in case the latter had a really hard time with a truly difficult situation.
He appears in the MMORPG Dungeons & Dragons Online, as a guide for your character to (spoilers) cross into Lolth's abode in the Immaterium as one of her sisters, The Spinner of Shadows, was one of the Demon Overlords of Eberron's history. He then takes you to Eveningstar to help the Purple Dragon Knights kick drow butt and stop both Faerûn and Eberron from becoming one with the Demonweb Pits.
Reason why Elminster doesn't do crap
There is a reason that is explained in the novels of why he doesn't kill every badass evil wizard and that is mainly the collateral damage.
Imagine two epic level wizards duking it out in your region - and no, they're not just going to buff themselves up, teleport in, cast time stop, and then cast four save-or-dies at the other guy, you mechanics-obsessed munchkin. Odds are they'll probably pull out several "big gun" spells, and the resulting carnage would send the entire region tits up and leave it a blasted wasteland for quite awhile. It would most likely be the equivalent of a nuclear bomb going off.
So yea, there is a reason that he doesn't man up and go after every BBEG in the setting.