I just want to know much it reduces for my dragonite.
50% So if an ice beam would usually do (around) 20Hp instead it will do (around) 10Hp but on the second turn if the Multiscale user is not on full Hp it doesnt work.
hazards can break this.
So you may want recovery moves and leftovers to really abuse Multiscale.
It halves the damage if the Pokemon has full HP,so,half the damage.As long as the Pokemon that has multiscale has full hp,it will take half the damage. Stealth rock and spikes can take away the effect of having half the damage taken.If you use roost or moonlight or recover,the multiscale effect will half the damage took.
I used this against a raichu that used thunder:10% damage!