First off, it may be far easier to keep him from getting to that point than it is to deal with it once he gets there.
Elves should be able to be pretty fast, and getting all five colors of mana to cast his commander should take some time. Deal enough damage fast enough, and even if he manages to get a couple indestructible blockers close to the end won't matter.
If he's relying on artifacts to help fix his colors, green definitely has ways to slow him down there. Naturalize is probably the canonical example, but there's plenty more. (Reclamation Sage is even an elf!)
You might be able to slow him down further by killing his commander before he can tutor for the other slivers he wants. Green's main way of doing that is fighting (think Savage Punch), which may take some work to set up, but if you have any big green creatures along with those elves it might work. And of course, if you're playing multiplayer, there's a good chance someone playing more removal-heavy colors will be happy to help out and destroy his commander.
If this really doesn't ever work out, you might want to ask whether the decks are really on the same power level. If he's got ridiculous mana fixing (fetch lands and dual lands and all) and you can't ever win anywhere near fast enough, it might mean that he needs to tone down his deck a bit or you need to push yours.
All that said, to answer your actual question... as you've noticed, green doesn't really have anything. It doesn't have much simple targeted removal, and it has even less that'll wipe the board. There's Ezuri's Predation from Commander 2015 (make 4/4s to fight everything) and a few things like Hurricane that deal damage to all creatures with flying, but not really anything that'll get past indestructible.
So if you really want to deal with this, you probably need the few colorless options:
But again, it might be better to try to overwhelm him before he can get going than to count on finding one of a small number of cards in your deck.