for long term, SSR weapon >>> event eidolon. Rahab isn't even good for pure water. The main quest SR water eidolon with 35% element up when maxed gives you more damage output, because Element attk up >>> character attk up at high weapon skill level.
Ablitity up seems to be nice. Unfornately the "BIG" up of maxed Rahab is only 20%. That means, if the abilty does 3x dmg, it will do 3.2x dmg. If you don't have any kami which uses offensive abilities often like shiva, HP boost would be a much better choice.
Only the rahab summon effect thunder resist up would be useful, once you will facing strong thunder bosses, but 10 turns....
And of course her harem.