Roaming boss monster appears and attacks with initiative? That is the timing you instantly use your bail-out items.
Stuck in a hole with roaming boss monster approaching (and the hole is not safe, merely cover from long-distance ranged attacks. boss can still use it mid-range due to its height, or dig to attack you from any direction in the hole), and no plan on how to deal with it? That is also the timing to use your bail out items.
That said, he should try a potshot with gasoline again. It worked oddly well for the smaller variants, so who knows, maybe it actually is super effective here too?