Aaaaand the derail strikes once again!
Back on topic, I once had a single serpent man armed with a wooden shield and spear destroy a fort.
ONE.SINGLE.SERPENT MAN.WITH WOODEN WEAPONS.AGAINST A METAL-CLAD MILITIA.
And he won.
He must have heard of
Âsax and thought that if a cave swallow man can do that, why can't he take out some dwarves.
My first megabeast in the current fort was a minotaur, which went down pretty easily. The second was a fire-breather, which did pretty much wipe out a 2-squad goblin ambush and a dwarven caravan. Meanwhile my dwarves whack-a-moled 4-5 thieves that showed up at around the same time. Thankfully the fire-breather was a 3-eyed maggot, so fleshy, and got wounded enough by the caravan guards that it died without my intervention. My current object of concern is a flesh-and-blood theropod (dinosaur/bird?) with wings and deadly spittle flying around the only cavern that it can actually get to my fort through, but it hasn't really shown an interest in doing so... yet. So yea. The difficult stuff are the ones that breathe fire/nasty syndrome spit/dust/extract, or even web... and/or are made out of tough inorganic materials like bronze or diamond.
Oh, and nether-cap (and I think fire imp leather?) shields would survive any grass-fire infernos too. The only situation in which nether-cap burns is in a wood furnace.