crud , this doesnt sound good. i have a donkey and a mare that dont like eachother.... the donkey is old , and the mare is small, so they havnt really hurt eachother YET. I ended up sending the donkey away for a month long "Holiday" ( jail) ... he came back minus the chip on his shoulder. and things improved. I will build 2 separate barns this summer so the donky will be with the ones that tolerate him.
I think it just takes time for one to back down, maybe you will have to split the pasture in half for a while. I had electric fencing all over the place until things settled...it was like a mouse maze. So I feel for YA.
I did things to make the Donky the weaker pasture pal, because i knew the mare would never really hurt him , but the donky , if allowed to be alpha could do damage. Some of the things I did to lessen the fighting were as follows
blankets, fly sheets, he hated them and would just pout in a corner
grazing muzzle
separation
special attention with the donkey away from the rest
and that "Holiday" with the farmer.
these things sort of let Willy know that he wasnt going to be in charge.
In this time where Willy couldnt pester Pearl , she learned how to defend herself , and gain some muscle.
It just works better for us to enforce 1 alpha mare , and discourage another wanting that role.
good luck