Caged necromancers are amazing. They're an extremely useful thing. Do not kill the necromancers! Capture them instead. They're immortal, and will not die of hunger or thirst.
Necromancers also cannot destroy buildings. Its very easy to make a shooting range of arena with endlessly reanimating undead. Just dig out a 10x10 room that is 3 levels high. On the upper level, alternate floor grates and constructed floor tiles. Necromancers can see through floor grates, but projectiles cannot move through them. A floor gate cannot support another floor grate, so you need to have constructed floor tiles in there.
On the second level, build a retractable bridge. Link it to a lever. This lets you toggle the reanimation whenever you want simply by pulling a lever.
On the bottom level of the room build a barracks and dump some corpses. Its best if the bridge is extended while dumping corpses. Once they're in place and you have your military ready, retract the bridge. The corpses will come to life and your military will begin its sparring lessons.
Note that you want to use small corpses. If you're dumping dragon, titan, whale, and draltha corpses in there, you're going to have a lot of fun. Start off with things like rabbits, chickens, and cats.