|Takumi and Clarisse have very similar stats but Takumi has 2 big advantages: Native access to Close Counter and variance in his stats. This, I find, can allow him to fill Clarisse's niche as a mass debuffer but do it better. Ideally you want +Spd for IV's but +Atk is a decent alternative
His weapon is going to be Clarisse's Bow+ and I recommend the speed bonus when refined to capitalize on both Takumi's good speed and the debuffing effect of the weapon itself
His assist can vary depending on his teammates and your playing style so just put whatever you feel works. As for a special I highly recommend Luna as it can pierce higher defense enemies. Moonbow is a decent alternative but since you should hopefully be doubling enemies more often than not Luna can charge at a fairly consistent rate.
His A and B skills work in tandem with one another. Close counter will allow him a counterattack in most situations and vantage is a way for him to very easily leave a parting gift if he's about to die. You'd be very surprised how much that last second debuff can turn the tide of a battle.
His C skill can either be his native Threaten Speed 3 as it helps with doubling but if you have a unit that does that already then Threaten Defense 3 is more advisable.
I recommend Savage Blow 3 for the seal as it gives a bonus when he attacks enemies on offense as they'll take a bit of chip damage as well as the Atk and Spd debuff from the weapon.
The idea behind this build is to allow Takumi to fill a unique niche better than the weapon's original holder and I've found it can work exceptionally well, especially with Gunnthra as a teammate.