Monster concept: living container.

[Seekeepeek]

Monster A
Monster size: medium
Monster type: when killed spawn 5 of type B
Monster spec: high damage. Low ac. Low hp. High Sr.

Monster B
Minster size: small
Monster type: dies when killed.
Monster spec: low to no dmg. High ac. Always kd on hit with super ab. Kill able by spells. Monk speed. Low hp

Conclusion:
Like slime monster that spawns smaller monsters when killed. These work alike that, however it require some tactic unless you want to be on your butt as type B kd you. While a type A kills you. So you did have to get all type A to near death before killing them or something.