Kinda surprised I didn't get any votes, but ok.
Psynergy
I think Aegi and Thund-I work the best against this core from everything I didn't nom lol. Aegi just takes whatever from these Pokes and does a lot back, OHKOing stuff if WP has been triggered which is often the case. I think a bulkier Thundy with Taunt would be better, at least for this core. Taunt from a flying type completely shuts down most Hippo, and then you chip it with HP Ice. More bulk would help get off two T-Waves on DNite(cuz first is probably getting healed with a Lum Berry,) and the loss in power doesn't matter if you can still deal with Hippo(in fact, Taunt is sometimes better since specially defensive Hippos take a Grass Knot and then Toxic or set Rocks.)
EDIT: Don't let this thread die before I get a single vote lol!