Playing [Lavos] is truly a test of whether you remember which buttons work together to imbue the right elements. You MUST remember to imbue at least one additional element on top of each cast to be truly effective, as then you would stack two elements per cast instead of one. If you can't remember which one you have stacked and/or just want to rely on muscle memory, Viral and Fire/Electric (non-Corpu$/Corpu$) are safe choices to put on any ability. When using Vial Rush though, it's best to stack Viral or Fire. Fire works best as you get to watch them do the fire dance in slow-mo.

[Lavos]'s abilities doesn't need too much STR to scale well. With the damage of Catalyze doubling for every elemental stack on enemies, it's best to soften them up with Viral and then Heat (yes I know Catalyze is heat based) to improve HP damage and reduce armor respectively. After sufficient stacking, Catalyze will trivialize even the toughest opponents in game. For reference, in order to deal the same amount of damage with just power STR, you need to waste a few more slots on STR mods and [Sacrifice] DUR or EFF just to catch up. Pressing more buttons, in this case, is justified. Or in the end you can just put Heat on Vial [Rush] and pretend to be a slightly different [Nezha].

Running maxed out RAN, however, is much better as you can then spread the love around.

Other notes:

Triple Umbral mod build - This is truly overkill, useless and a waste of time and energy and forma. As the video will prove, even at 108% STR you are already killing anything that you can be bothered with.

Higher EFF - This is calculated to be slightly suboptimal. While more EFF can reset your cooldowns, you are instead stuck with the 10s requirement of Transmutation Probe. During that time, you would have done two casts of Vial [Rush], which should be your bread, butter, chicken and avocado instead. Another thing to note is that even at max EFF, you still need to hit 12 enemies with it to reset the cooldown of Catalyze which is a waste of three and a half slots (since you need two EFF mods and one DUR mod to counteract the negatives).

More survivability - if you're getting hit after putting 10 Cold stacks on the enemy, I suggest that you check the batteries on your wireless mouse and keyboard.

Arcanes - Grace and [Guardian] are safe choices, as are Aegis and Barrier (if you want to go down the Shields route). Since he has no Energy requirements, Energize isn't needed unless your demanding soulmate teammate is a power hog, in which case I would like to introduce you to [Trinity] instead.