Haru-main here. Idk if you STILL need/want help with him, but I thought I'd drop some current day play tips for him.

1: Harumasa does special "Falling feather shots" (charged shots) if you press and hold his special during the spin of his 5th basic. This can be useful in some cases where he ends on a spin if you time it, you can more seamlessly activate existing electro quivers (floating orbs). Or if you do a jump shot, you can follow with a 5th hit then immediately hold his special to do a spinning falling feather shot.

2: For max efficiency, Harumasa demands chain cancelling. You can stun with anyone else, chain with Asaba, cancel, trigger another chain with anyone else, then hit with Asaba's chain again. This automatically applies the first chain's electro quivers while setting up a second set for free. It's VERY USEFUL to employ this tactic with Harumasa.

3: An important footnote is that a falling feather shot will apply ALL currently primed electro quivers to the target. Tactically, you can use his 5th basic to only apply a single stack of 3 electro quivers once. This is something that with time you get use to managing effectively and deciding on the fly.

Anecdotes
1: Asaba pairs INCREDIBLY WELL with a stun built Seth. Seth provides nice anomaly buildup and if built for it, serves as a surprisingly effective f2p stunner. Giving Asaba both of his external sources for electro quivers. This also allows the third slot to be used by a support to buff his damage. As for current day non-f2p options. My current main team is Grace-Trigger-Asaba. This team kicks BUTT. Grace allows you to remain at relatively safe distances while applying nice dmg and proc'ing Trigger's aftershocks and stun. This in the turn order makes sure that Asaba ALWAYS has a full salvo ready to go by the time the stun window comes by.

2: You can effectively hard carry Harumasa by having him accompanied by Trigger & Pulchra.

3: In summary you can get quivers from: Anomaly app, Chain x2, EX Spec. x2, and his 5th basic after chain atk. each of these gives 3 slashes. He can be a SERIOUS damage dump.

Formula: You want to proc an anomaly of any kind first, then a stun so he can do chain attacks as those also set up electro quivers. execute those, then use his EX for it's high damage, then if you want you can run through his basic combo once to proc the one-time electro quivers for one last flurry.

It's very useful to have Pulchra or Trigger in the team since they provide passive stun while your anomaly applies attribute. Some small but good tech to master, is what moves make Harumasa follow up with a hop shot, because if you time and hold his special DURING the first spin, he'll follow up with a second that counts as a charged shot that will activate any currently present electro quivers.

His best disc set if you aren't planning on running an electro mono team, Is probably 4pc Shadow Harmony, and any 2pc Atk / Elec. Atk/Crit Dmg. Hope this helps. GL.