For Amara, using Psychostabber with the Orb damage anoint works very well against most bosses (the only ones it doesnt work on are Hemo/Vermi and 2nd stage of the Vaulthalla boss but that’s where the fish slap/guardian angel/dragon jr. Combo comes in).
It helps to have a point in the Find Your Centre skill and one in the melee skill in the red tree (purple tree melee skills are nice to have but not necessary).
In addition, revolter with ASS bonus seems to pass to the Orb as well, so that gives you even more boost.
And if you can find a Breaker mod with an extra skill point for Find Your Centre and either Action Sklll Damage or Melee Damage passives (or both) that is even better.
Relic is a cherry on top. If you have all of the above you wont even need to have any melee related relics or its passives but naturally that helps too. So if you got a victory rush, a toboggan or a dragon jr. - sweet. If not, dont sweat it. You should still be able to handle the bosses with minimal trouble.
My recommendation is to have blue tree maxed out so you can trigger the Orb twice, should it be necessary.
Otherwise, hit the orb once, hit the orb twice and unless the Boss runs around like a chicken on fire, the elemental damage it does should literally melt everything near it in few seconds. A 3rd hit is sometimes needed.