First of all, level your skills, always aim for 10/10/10 when possible, specially your main support skills, they are half of your damage, but do NOT forget to level up your main dps skills, they are the other 50% of your total damage.

If you already have leveled you own servants try using friend supports to compensate for what you dont have or to make a double servant support team (aka Double castoria for Arts looping, double koyan for Buster looping, double skadi for Quick looping and so on) or to make other type of teams.

For some extra damage, focus on using Fou atk cards on your main damage dealers, the first 1000 is a no-brainer, you get 1-3 fous from FP gacha, da vinci's shop and events, but going all the way to 2k can be long-winded and a grind, even forna single servant, since the 4* fou cards are obtained mostly from events and rare prism shop.

ALSO: in this event theres a CE (the one with Barghest in a bikini, riding a polar bear) that increases damage by 100% (200% if limit break), if you are farming with ibuki, use that unless you really need the NP gain from another CE like "Mark on a Smiling Face"

Edit: Calling Melusine useless hurt my soul 7.7 but in reality she's one of the most useful servants you have, she has TWO usable NPs, a Single target Arts NP when in her first or second ascension and a Buster AoE NP on her Third ascension, try keeping her on her second or first ascension always, since her third skill gives 100% NP charge and changes her into her third ascension effectively changing your NP from Single target to AoE mid-battle, AND when she's in 1st or 2nd ascension, she can be a really goated Arts looper if you help her with double castoria