Phasecasting

There is a special technique to getting more damage out of your Phasecasts, more specifically Reverberation. This does not work with Tandava.

When you cast at a surface, your Phasecast "splats" against it, whether it's a wall or floor. This is a separate instance of damage from the cast itself, and it deals splash damage. So if an enemy is backed up against a wall and you cast at them, they will take two instances of damage: the first when the Phasecast passes through them, and again when it splats against the wall behind them.

This can be done to an enemy that doesn't have a wall behind them, as long as the enemy is on the ground. You can splat the ground just under their toes, and force the enemy to get hit twice. This is especially important for Reverberation, because Reverb's inherent ability is that it gains damage as it passes through enemies. So that second instance, the splat on the ground, will be stronger not only because of splash damage, but because the Reverb buffed itself. This is on top of Reverb's base damage being much higher than the other Phasecast action skills.

Watch as a normal Reverb cast is compared to this special Reverb technique:

This is especially useful for casting builds vs bosses. Tandava is great for mobbing, but falls far short on single target damage vs Reverberation with this technique.

TANDAVA

Tandava does have its own special ability, though (along with Phaseslam). Its ability to wrap around or penetrate hit boxes allows you to hit enemies behind cover. This ability also allows it to multi-hit a couple of bosses that have multiple hit boxes.

Wotan's bottom half actually has 5 hit boxes: one for Wotan himself, and one for each of his legs. Tandava's ability to penetrate hit boxes allows it to hit Wotan for 5 times the damage. Check out this screenshot:

This is a single Tandava cast at Wotan. Note the four damage numbers up top, one for each leg. There's also an obscured one in the middle near the bottom.

Another boss that has multiple hit boxes that can be exploited by Tandava is Blue Fire at the end of Slaughterstar 3000. There may be more.

HEMOVOROUS & VERMIVOROUS

Unfortunately, Gearbox coded Hemo and Vermi in a way that no form of Phasecast makes direct contact with them. If you send a Phasecast at them, it will simply pass on through as if they weren't there. The only way to damage them is to splash Tandava on the ground near them, and hope they are within the damage radius. This can be tricky when they are in their flying stages.