Slight hijack but one thing I truly hope changes sometime soon (I recall Toady talking about it once..) is cage traps. Its absurd that a wooden cage trap made by a person of zero skill in mechanics or woodworking can hold any creature in the game regardless.. Frankly there should be a strength rating to the material and possibly skill that goes into making the trap in terms of deciding if the creature inside is held in place, and some creatures (such as megabeasts) should be impossible (or at least nearly so) to hold in a trap.
I guess you could say 'just don't use them', and for a long time I didn't for this reason (and still don't spam them), but I do like cage traps for trapping live animals for breeding and for trapping live invaders for.. Errr.. Training.. However nothing sucks the fun and adrenalin out of a situation to see "A great and powerful monster/dragon/giant/etc has arrived!" only to have it get itself trapped in a cheap wooden trap that should have a hard time holding a deer in place moments later.
hijack over
In answer to the OP question, I'd go with archers if possible and dwarves with shields (that can block the fire attack), traps also do work as mentioned but if he's already here then they probably need to already be in place. As others said, the problem with fire is that (depending on your biome) it tends to destroy huge amounts (if not all) of the life (plants, animals, trees, etc) it touches and it spreads very fast, so (again depending how your biome is layed out) don't expect to gather much wood in the near future.
-MB