Same type buffs (i.e. Charisma and Waver's ATK buff) are additive to each other, and different buff types are multiplicative.
So two 25% ATK buffs total up to 150% damage (assuming a 100% base of course). If you instead had a 25% ATK together with a 25% buster, then buster cards would deal 156% which is slightly better.
It's generally best to have as many different types of buffs as possible, and optimal gains would be adding more of whichever buff type is lowest. In an example of 40 ATK, 30 buster, and 20 NP strength (~218%), if you add 10% more ATK then it would be 234%, adding 10% more buster instead would be 235%, and adding 10% more NP strength instead would be 236%. A small difference, but a meaningful one in the grand scheme of things. If all of those were piled into solely ATK instead of being varied (so 100% ATK) it would just be a measly 200% damage compared to the other potential options.
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Damage is variable, yes. The final total of all damage calculations is subject to a plus/minus 10% random multiplier. The difference in damage is much more pronounced at higher amounts since it is a straight multiplier to the final total. Something that calculates to 1 million damage can deal anything from 900k to 1.1mil, a variance of 200k damage. But something that calculates 10k will either do as low as 9k or as high as 11k, only a small 2k difference.
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Aerial Drive and Mana Burst would be additive to each other, Golden Sumo would be additive with Charisma. The NP Strength on Aerial Drive is multiplicative with Mana Burst and Charisma, since it's a third buff type altogether, so usually that pulls it ahead of Sumo for specifically buster type NPs.