In what kind of boss fight? Melee builds aren't really for soloing invincibles, as for regular bosses most of them should be easy enough.
And you really need to be active on the weapon switches. It's not like Zzzz Bee shield where you just use the best multi-shot weapon until you're out of ammo, then switch to the next one, then the next one...
The Thumper does explosive damage when drained. With the Rubi equipped, the explosive damage is healing you to full life (or very close to full) with every attack. If the Thumper has any shields, you won't get life back through the Rubi. I think that's part of the TC's confusion.
Any Rubi. Any level. You just need it equipped and it will heal you for 12% of ANY elemental damage you deal., from any source The Thumper is dealing insane amounts of explosive damage...when drained. Hence Grim isn't helpful because each melee attack is refilling a small portion of your shields, and the Thumper is no longer drained.