To elaborate a little:
What you were (roughly) hoping for with Spear Dynamo:
- Damage = ATK * SkillMultiplier * SpearDynamoMultiplier (this would be awesome!)
What you are (roughly) getting:
- Damage = ATK * (SkillMultiplier + SpearDynamoMultiplier) (this is not so awesome)
When auto attacking, Spear Dynamo's 25% ATK boost can easily translate into a 25% damage boost, because auto attack damage is a direct function of your ATK. Boost ATK by 25%, get ~25% more damage.1
But, when you're attacking with a skill, you now have the skill's % ATK multiplier in the mix - and Spear Dynamo's 25% boost doesn't multiply that multiplier, it just (effectively) gets added to it before the latter is multiplied against your ATK. So, Spear Dynamo will underwhelm when used with a skill that already has a huge ATK multiplier, because the % ATK boost Spear Dynamo provides is now just a "drop in the ocean" of the total multiplier provided by Spear Dynamo and your skill when added together. Relatively, it's a small boost. E.g., suppose you are using a skill with a 1500% ATK multiplier. Spear Dynamo doesn't increase that multiplier to 1875%, it increases it to 1525%. Or, in other words, an increase in your total ATK multiplier of ~1.67% - and roughly the same boost to final damage.
(1) THE FINE PRINT: Actual results may vary depending on what buffs you have, which can interact with Spear Dynamo in idiosyncratic ways (e.g., Aura Blade).
Edited by INREM, 16 August 2018 - 08:35 AM.