You can farm ultimate and hope! I got 2 rainbow chests, one with Rahab and second with water orb.
I see usually 2-3 rainbow chests per event (during garuda was my record 5 rainbow chests), but then again I am drinking half potions like crazy during those events.
If you are using water team then, in my opinion, you should get water orb and gun to max. 1 SSR weapon that you can't max lb won't do much if you already have maxed version.