How do I scale Plage Bearer?

Besides prism of belief (+3), crown of victory (+1) support gems (+2), gemling (+1) and reg passive tree (+1).

Is there other ways to add levels to Plague Bearer? The scaling is insane on it
It's a buff not a spell so +# to spells doesn't work on it sadly.

I'm really enjoying building around it. Using it + corpsewade is a joy.