[Thermite Rounds] can be replaced with [Vigilante Armaments] for slightly more sustained damage.

Edit:
So I bought this and forma'd it 4 times for this build while lich hunting and in ESO. Aftere a short while, I tried to change the corrosive to viral and still add heat, but going Rime/Malig and Thermite ended up making Viral/Radiation with Radiation being higher then Viral. Still, it wasn't useless. I was causing all kinds of chaos with mad Rad procs on clusters of enemies that turned on each other.

Through trial and error, I found that I preferred Viral/Electric to Corrosive/Heat or Viral/Rad. Thing is, the status is so high that when you hit an enemy, you immediately stun-lock them (with electricity) as you hold the beam on them until they die. And if they have a crowd around them, you can stun them too with the electricity chaining. Neat. Combined with Viral, it's pretty deadly. I was holding off huge groups with this, stunning them all.

Also, the range is fantastic. I figure it's 49 meters, which is TWICE the range of all of my other beam weapons. This thing is a beast.

And THEN, after forma'ing, I discovered the weird cube-grenade secondary fire! (!!) I'm very impressed with this weapon. Definitely get it from Baro when you can.