The best passives for Skutlass Ignis depend on the job you give it. Skutlass Ignis is Paldeck #128, a Water / Fire Pal with Kindling 3, Lumbering 3, Watering 2, Gathering 2 — the sets below are the community-consensus best-in-slot for each role it can fill.

LegendDemon GodMuscleheadLord of the Sea
Legend, Demon God and Musclehead stack raw attack; Lord of the Sea adds 20% Water damage on top.
ArtisanSeriousLuckyWork Slave
Maximum work speed. Work Slave trades attack a base worker never uses; swap for Nocturnal on day-sleepers if sanity drops.
SwiftRunnerNimbleLegend
Every movement-speed passive stacks; Legend adds 15% more plus combat stats.
A perfect Skutlass Ignis also wants 100/100/100 IVs (breeding averages the parents with a bonus, so high-IV Skutlass Ignis parents compound) and 4-star condensation — feeding it spare copies raises this Water / Fire Pal's stats and every work suitability a full level. Craft-speed and attack souls from the Statue of Power stack on top. The planner routes these passives from pals you own; see how to breed Skutlass Ignis for parent combos.
Breed this Skutlass Ignis build from your save →