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Response: After you engage an enemy, exhaust a Scout or Ranger character to place X progress tokens on a location. X is the engaged enemy's printed [threat].

Can I play Expert Trackers against an enemy immune to player card effects, as long as the location is not immune to player card effects?

I'd say yes because you're not choosing the enemy, it just happened to engage you. You're choosing the location instead.

That's what I thought, thanks. Just wanted confirmation.