Hi again guys,
Abyssalcraft also adds some bosses. It does require some in-depth commitment though.
Infernal mobs adds a few enchantments to mobs' strength, so they might be faster, or apply wither effect, or slow you down etc. Although not boss mod, this could be used to make an ordinary mob into boss (or make an existing boss even more challenging, that is if you have a deathwish c: )