Ok i'm going to start out of order but i'll keep it short and to the point.
Ultimate Beast/Mega Titan; these things take 4x2 squares (2 z levels high, 4 square levels). These are the rarest, and can not enter your fort unless you have a big enough entrance (note some can crawl which lowers them to 1 z, but they go much slower). These MUST BE DEFEATED WITH SIEGE WEAPONS. Yep, so when you are gettin late in the game and realize uh oh i could be attacked by one of these; it'll be time to make up wall after wall (walls slow them down as they destroy them) and you just have to get like 3 ballistics ready to constantly fire at them. If you take out there legs, or they are crawling; then melee can attack them. until then dwarfs get instant KO stepped on. Also archers can help attack too; but ballistics are really required to win this fight. BTW ballistics don't have to be made from cotton candy but dwarf equipment to really matter should be. Also when a MT/UB spawns they are peaceful, they are just on a set path.
Say they spawn at the south end, a random point on the WEST,EAST, or NORTH end will be created as there end spot. if you are unlucky and they are walkin through your fortress then good luck
otherwise they may just walk by if you leave them be (as in you don't have above ground fortress just a lil hole in the ground.)
3. Heaven/Death Angels, building upwards to 100z levels will pierce the clouds; and once you pierce the clouds in the same way you find a cavern, you will find a floating cities. they will ignore you until you pierce these clouds, once you do same thing as going to clown college will happen. Note that there is cotton candy weapons and armor waiting to be grabbed by dwarfs, also other rare items that haven't even been thought up yet!
1. Alchemy/dwarf wizardry; now whether or not a game with hell is considered realistic; we can go by one or the other. But same end results (some what). Alchemy requires ingredients to cast it's spells, where as wizardry will fatigue a dwarf (as they gain higher skill level in magic/wizardry they don't tire as easy). Common spells such as fires which starts fires to attack thy elves with. also create water/ and move earth which would be channeling or creating ramps from a distance so you can do those tricky traps that would otherwise kill your dwarf. Also using a study from chairs/thrones and/or adding the item bookshelf would be where they can train at. Also they would require a new workshop which is alchemist chemlab, or wizard school
. So main point isn't to cast magic in battle, but to get a few cool tricks for more !fun!