A disgraced veteran ex-firefighter attempts to redeem his honor by volunteering to lead a team of greenhorns to combat a forest fire of epic proportions that no amount experience and training could prepare them for the dangers they’re about to face.

You’re on the right track! A couple of things jumped out at me initially:

Since you already have “ex-firefighter” in the logline, you don’t need “veteran” since previous experience is already implied.

You could condense what you have into something like “A disgraced ex-firefighter seeks redemption by leading a team of green horns against a blaze that goes beyond anything they have ever faced.” However, I think I have a better suggestion:

Instead of trying to condense “to combat a forest fire of epic proportions that no amount of experience and training could prepare them for the dangers that they’re about to face” (which to be honest seems like an overused phrase), instead try to focus in on what makes this forest fire unique or different, and therefore, more dangerous, than the other forest fires out there.

For instance, you mentioned that he tries to redeem himself as a volunteer firefighter, so maybe explore what happened to the crew of active duty firefighters that led to them calling in volunteers to heighten the danger and make the stakes more specific. Obviously large forest fires will often warrant them calling in volunteers just to help contain it in general. However, maybe there’s a more specific reason that you can give it.

You mentioned how this is a team of green horns so maybe they don’t yet understand fire safety. Maybe they are just local townsfolk themselves, but when the active duty crew are missing in action, it’s up to them to finish the job.

I hope that helps! 🙂