This is a build for both bossing and mobbing, makes use of very high action skill damage with Phasecast as well as tons of general Weapon Damage for bosses.
Most skills are self explanatory, go down the Mystical Assault tree to get Avatar, picking up the best skills along the way. Five points in Do Harm to maximize action skill damage, which I think is more valuable than anything else, but you can switch the 2 extra points into Alacrity or Restless if you want. Only 1 point in Remnant because the base damage of it doesn't matter, all of its damage comes from overkill. The three points in Awakening are mainly to make Do Harm even stronger, to maximize action skill damage, but you could move those to From Rest if you desire.
The rest of the points are dedicated to getting down to Sustainment, so some of them are wasted. One point in Infusion is important, so you can proc dots if you ever use non elemental guns to gain Rush stacks. The five points in Anima are for abusing Elemental Projector relics, as it makes your self sustained DOTs last longer. Have to put one point somewhere to get down further, so you can put it in either Wildfire or Illuminated Fist, both are useless. One point in Catharsis not for the damage, but for the knockback it deals to enemies. Only have 2 points left for Sustainment, but that is easily enough to survive.
You always want Phasecast anoint on your gear, it is ludicrously overpowered. With your short cooldowns and Avatar, you will basically always have it active.
Heavily Recommended / Required Gear
Phasezerker Classmod - Cornerstone of the build, it is what allows you to stack Do Harm and do insane damage with your action skill. I recommend getting one without points in Conflux, as they are wasted and all it does is make you give yourself multiple DOTs at once when abusing Elemental Projector Artifacts.
Elemental Projector Artifact - Incredibly overpowered at the moment, and you can make the bonus last even longer with Anima. The best you can hope for, in my opinion, is an Elemental Projector Otto Idol, for the health back on kill, along with +40% mag size.
Band of Sitorak - An incredible shield for boss killing, you can keep it down for a long periods of time with the extended DOTs you put on yourself with Anima. If you don't know where to get this, here is a guide. https://www.youtube.com/watch?v=mKs0Lt3eYeE
Hellwalker - Deep Well + 40% Mag Size bonus Artifact gets you 2 shots out of it, very powerful gun.
Brainstormer - Tempest boosts Shock Damage a ton, you can spec 5 points into Infusion if mainly using it.
Cutsman - Insane damage, just a crazy weapon in general.
Lyuda - Again, insane damage, maybe the best gun in the game right now. Good regardless of your build or particular synergies with it.
Hex - Amara doesn't have any grenade regen or grenade damage, so this won't do much damage, but you can use it to heal with Sustainment.
Transformer - You don't have any synergies with it, but it is a very good shield in general.
King's / Queen's Call - Nerfed heavily, but still very usable. Essentially infinite ammo if you are accurate, and it beneits a lot from your increased mag size.
Antimatter Rifle - A strange one, but one of my favorites. Antimatter Rifles are Maliwan Snipers with the Alien Barrel. Ever since the general Sniper and Maliwan weapon buff, these have become incredibly powerful. You can easily one shot a lot of mobs with it, which is especially nice with Remnant.
Everything else. Amara can use anything.
Action Skill / Augment
Reverberation Phasecast is incredible for mobbing, especially with stacking Remnant. It has almost no downsides to normal phasecast. Soul Sap is almost useless but still preferred, because all of Amara's other Augments either decrease Action Skill damage, increase cooldown, or both.
Just switch your element between Fire and Shock depending on what you need, although fire is almost always preferred, as a good Brainstormer will an entire room of shields instantly.