What damage bonus does Infergon get against debuffs?

The useful part is that Infergon supplies its own condition. Lava Breath applies eight fire stacks in a single cast, which is already past the threshold, so you are not waiting on a teammate or a slow build-up to turn the bonus on. Hot Breath does the same thing to everything in a large area in front of it, eight stacks of Fire Debuff at once, which means the bonus applies to a whole group rather than one target.

That gives Infergon a clear opening order. Land the stacking skill first, then spend everything else while the debuff is up. Leading with your damage before the stacks are on the target throws away the multiplier on your best hits.

The debuff bonus is separate from elemental matchup. Fire attacks already hit Ice and Grass types harder under the affinity system, and stacking fire debuffs does not change that. Against an Ice or Grass enemy you get both at once, which is where Infergon looks strongest. Against something that resists fire, the debuff bonus still works, but you are climbing out of a hole first. Infergon also deals more damage to targets already suffering fire status effects, so the fire theme runs through the whole kit rather than sitting on one skill.

Not settled: Whether exactly five stacks trips the bonus or you need to be above five is not settled. Since Infergon's own breath skills apply eight at a time, the distinction rarely comes up in play.