I have to deal with six megabeasts.
-The first two are trapped in my trap alley. Both are immune to drowning, magma, traps, and self-regenerate faster than my military can throw bolts at them. To get rid of the pests, I have sent 20 dwarves into intensive military training and will unleash them once they become legendary wrestlers, legendary shield users, expert armor users (and fully clad), and hold an expert mastery in at least one weapon. Even then, I am not certain they will be able to take down one of the beasts, let alone two, as both have obliterated multiple simultaneous goblin and skalassi ambushes with disconcerting nonchalance.
-The third and fourth are impossibly agile flying creatures that almost always one-hit-kill and are able to out-tank an attack from 20 simultaneous invaders. That's right, they regenerate faster than 20 goblins attacking them at once. They move at incredible speed, but fortunately seem to need air - thus, a drowning chamber should get rid of them, so long as my drawbridges rise quickly enough.
-The fifth is a gray dragon, flying and amazingly strong. It has destroyed five migrant waves and just as many caravans so far. It is slow, fortunately enough, and its regenerating ability seems rather limited, so a good bolt barrage ought to get rid of it.
-The last is a death tyrant equipped with fifty tentacles. A complete unknown. It hasn't fought against anything yet, thus I cannot assess its fighting ability.
Meanwhile, I prepare the drowning chamber in order to make up for those megabeasts' ability to completely avoid traps. At least this will deal with a portion of them. Some of the water from the underground river will be redirected to reservoirs in order to begin a tower-cap farm, seeing as current production is nowhere near what I wish it was.
Eventually, I'll be able to keep the entrance sealed and thus fully secure all dwarves from the outside world.
Meanwhile, the megabeasts do a fine job at routing sieges.