i normally train my marksdwarves on forgotten beasts that are inorganic and heavy. Even a forgotten beast made of vomit wont completely die unless you melee it and cut its head off or bisect it. I can see how this would work for a early fort without any beasts yet, but with necromancers, though i tend to use a different design than the one you posted:
lvl1: lvl2:
W W W W W F F F F F
W W F F
W N W F F
W W F F
W W W W W F F F F F
W=wall, F=fortification, the N necromancer is chained so dwarves ignore it and it will also not be directly hostile to dwarves, but can still revive the dead. Level 2 is surrounded by another ring of walls 1 tile behind each fortification so that marksdwarves are forced to hug the fortifications and shoot through it. The necromancer is removed by pulling a lever to the chain and opening a passage to some cage traps, as it tries to path off the map. To get corpses inside either pit live undead from above behind a hatch or dump a bit of refuse onto the hatch above and pull a lever linked to the hatch.
another way to do this without necromancers or a forgotten beast is to capture a sturdy cavern creature like a blind cave ogre, or even use one of your wagon animals. Chain it outside until huskifying weather turns it into a husk, then pit it in a similar pit. Husks wont die unless bisected or beheaded.