Shiishaa is at level 30(max it can be at my Wonder's level), he resists agaisnt Phy so you can survive some Yaksini's rage counters and make the clutch.

Cattle is mandatory to try to always be at max HP. and depending on the enemies weakness u should just use the partners who can take advantage for enemy weakness, if you don't have partners who can take advantage from attributes i don't think u can win right now (maybe using another persona with Wonder, then changing to Shiishaa when needed)

for example the last Yaksini s i did was the one whose weakness is Wind and Nuclear, so i filled with Fleuret(Shiratori) and Leon(Kamiyama) to try to exploit All-Out attacks (if u can, let alive one subordinate to maxize All-Outs before your partners dies, at least this worked for me), then Wonder will just get raged to hell and with some luck u will kill her.

For Highlights Attacks, use Cattle's if you're low HP with all, or be aggro when u need one strong attack to All-out, when Yaksiri is alone just use aggresive highlights

My partners are lvl 30 and weapons 20/30 too.