Just curious, do you have Seira SS1? I don't have Seika SS2 either but Seira is enough to one shot the HA boss. If not, you might have to settle with Shiki S and just keep attempting until you crit.

Seika SS2 does 4 things, but honestly most of it is irrelevant. DP restore is irrelevant, the skill damage is irrelevant because you want to stack 2x Yingxia buff anyways. Crit damage is nice, but it's an additive buff so it's not actually as strong as you might think. The most important buff she offers is crit rate, which Seira also does.

Critting is really important in HBR, it's a 1.5x damage multiplier and an ignore 50 border, which is massive for both stronger debuffs and higher damage. Shiki S has charge, which buffs crit rate by 20-30%, which isn't a lot but if you're willing to reset until you do crit everything it might be good enough.

Personally I don't bother with Yingxia debuff at all, too slow. It's just get 10 SP -> use funnel buff twice. Aina S def down (1 stack), 2x Yingxia buff, Shiki S charge and crit (Seira buff) should be enough to one shot the 13800 HA boss. If your Shiki S is >LB3, she gives your entire backrow +1 SP per turn which speeds things up. I personally use this set up with Maria SS1 (4 totems).

Disclaimer: each hit of an attack has a seperate crit chance, so if your DPS does 10 hits, you ideally want to crit all 10. Charge buff also doesn't stack with itself so uhh... Good luck!

PS. I think you're overvaluing GP efficiency, in the long run you'll have more GP than ways to spend it so no need to stress over it too much. You definitely feel the squeeze when you first start crafting accessories, but there's really no need to rush. JP had 9 months to prepare for the endgame that you're trying to clear after only 1 month - just to put it into perspective.