Soul-collectors.

Normally quite peaceful, these colorful insects transform into fiends. There are three classes of fiends.

First class: Melee. They turn into spidery creatures the size of large dogs, with blades for legs. Fast, quick, efficient, and with more eyes than they know what to do with. They're impossible to sneak up on. However, they are effectively glass cannons: a solid hit will take them out.

Second class: Aerial attackers. They turn into large half-lizard half-bird creatures (Wyverns) that can shoot devastating beams of energy from their mouths. They're sturdier with increased evasion abilities, but only two eyes, so they can be caught unawares.

Third class: Tanks. They turn into ten-foot-tall chimeras. They can also shoot beams of energy from their mouths, but also have incredible physical strength. They can crush and lift boulders, crack the earth with a determined stomp, breathe fire, and cast elemental magic. They have low evasion, but are extremely difficult to disable, and ultimately very powerful.

In their insect form, they are incredibly vulnerable. However, they can cast complex and creative rune-spells in this form, which may actually be worse for the enemy. Additionally, if they die, they can be carried to the flowers of the World Tree, which will reincarnate them.

I don't mean to brag, but considering there are thousands upon thousands of these creatures in my world, just hanging around and flying together and doing their jobs, I think they'd win. Just one of these is enough to take out an entire kingdom if sufficiently and continuously provoked.

The only downside is that if you stop attacking them and back off, and avoid the next few blows, they'll slow down and stop to watch to see if you're about to retreat. If they're convinced you'll leave them alone, they'll just shrug, de-escalate, and scuttle off. They're never the initiators of aggression. This means enemies could strategically take advantage of these calm-downs and lulls, like waiting for a porcupine to unfurl just enough to gut the underbelly. But if you provoke them and keep provoking them, then not even the gods can help you.