Leave It Up to the Players
If you're running a West Marches-style game, do you necessarily want to be balancing encounters?
Quoting from one of Ben Robbins' posts:
[A]ppear passive — The world may be active, but you the GM should appear
to be passive. You’re not killing the party, the dire wolf is. It’s
not you, it’s the world. Encourage the players to take action, but
leave the choices up to them. Rolling dice in the open helps a lot.
The sandbox game really demands that you remain neutral about what the
players do. It’s their decisions that will get them killed or grant
them fame and victory, not yours. That’s the whole idea.
(From West Marches: Running Your Own)
For instance, perhaps there're rumors of a young red dragon somewhere on the slopes of Dark Smoke Mountain. From a west-marchian point of view, you could argue that it's the players' job to figure out how to investigate and perhaps conquer that challenge with the players available. Maybe they decide to use the level 8 tank as bait and then all the lower-levels attack from cover or buff the tank or loot the cave while the dragon has some level-8 lunch.
Solve It With Them
You can even join in discussing the solution. To quote Ben again:
Because the rules were well-documented and clear, there were lots of
times when West Marches combats would become fascinating (albeit
life-threatening) tactical puzzles for everyone at the table. We would
all gaze down at the battle map (me included!) and ponder possible
(From West Marches: Secrets & Answers (part 1))
There you are at the table as the PCs are discussing the pile of loot the red dragon no doubt has. You say, "Wow, holy smokes (ha ha) guys, how are you going to keep all you level 3s from becoming shiskabobs? I mean, hey, you guys do whatever you want, doesn't matter to me, but if you just form a skirmish line, and whack away, you low-level guys aren't going to make it. Ah, maybe you're thinking with all that loot the cleric at Joe's Temple will be inspired to cast a few raise deads. Sounds like a plan. So what do you do?"
For that matter, maybe they'll decide the red dragon is too tough and go pick on some goblins, and the player of the level 8 will come in with a level 1 instead.
Regardless, to me, the west-marchian answer is to describe the world, then let the players figure it out.