[Nerfix]

Funny, I just fought against Agartha and their Seal Guards, which weren't that large in numbers I admit, died in droves again my Ambitates...

Onis might be a different beast, but I'd just Curse and Horror Mark them.

EDIT: If morale is bringing you problems, mix Ambis with Standards. I use Carnutes only when I must quickly bring reinforcements over forest provinces.

For erly expansions I used their Javelineers. Sure they die a lot, but are also easily replaced.