Fusion Level — What to Fuse & When

1. The base carries the level

Fusion keeps the base's role and re-rolls its stats with the donor's talent. The output's power floor is set by the base's current level — so the side that deserves your levels is the base, not the donor. Raise the base to match your active roster, then fuse on top of it.

2. Don't level dump the donor

A donor supplies the locked trait. Leveling it up mostly wastes income you could have put into feeders or the next egg — see the money guide for where Cash actually compounds. Fusion cost scales with the fighters you bring, so an over-leveled donor makes the fuse pricier without making the output better.

3. What actually raises the output

Three things decide the fused result: the base level, the quality of the trait you lock, and whether the outcome beats your current best. A strong trait from a Thunder Egg donor on a level-appropriate base is the cleanest mid-tier breakthrough — check the special abilities guide for which traits are worth locking.

4. Fusion level across a run

Early on, fusing a mid-level base with a common donor widens your roster cheaply. As you climb, save fusions for the push moments — when the current floor walls you and the trait you can lock would break through. Keep the loop funded with the combat hub's income routes, and save the pricey fusions for right before a rebirth.

5. Common fusion mistakes

Two mistakes account for most wasted fusions: raising the donor, and fusing before deciding the target trait. The donor only has to exist — its level is irrelevant to the output. And a fuse without a chosen target trait is a coin flip you are paying for, because the result is compared against nothing. Decide what a "win" looks like before you spend, then check the output against your current best.