What does Shrouded Mist do to Nimbi Burst?

Outside the mist, Nimbi Burst gives the whole team 15% more damage and 15% more break efficiency for 20 seconds. Cast while Turbo is Shrouded in Mist, both numbers become 30%. That doubles the skill's value against anything you are trying to break.

The cooldown is 18 seconds against a 20-second buff, so you can keep Nimbi Burst running permanently. The real question is how many of those casts land inside the mist. Shrouded in Mist triggers at four stacks, and Turbo builds those stacks passively while it is off the field, reaching the state after 20 seconds. Bench Turbo, let the stacks fill, then swap in and cast. Firing the burst on cooldown regardless just gives you the 15% version.

The damage part is a flat increase that does not stack with other effects, so the 30% figure replaces the 15% rather than adding to another boost of the same kind.

The same rule on Prismana Turbo

Prismana Turbo's Nebula Burst also gets stronger in the mist. Its Dark damage boost for allies goes up there, and it reaches all nearby allies. So the same habit applies: activate Shrouded in Mist first, then cast the burst right after it.

Not settled: How Shrouded in Mist activates is not settled. It is described both as triggering after four basic attacks and as triggering when a meter filled by dealing damage is full. That changes how you time a boosted Nimbi Burst while Turbo is on the field.