It depends on how your teams compare to each other. If you end up with a team that's far stronger of the others, that points towards focusing on that team more. The thing is, as F2P, the choice isn't really in your hands. You're functionally at the whims of the RNG to dictate how strong your teams are at any given time. One day it can look like you should be focusing on water due to Rudra. Another day, the RNG may throw more water stuff your way further reinforcing the idea to focus on water. Or, the RNG decides that you get some nice stuff... in another element or two.
I'm F2P myself, and the distribution of RNG for me has such been that, I don't have any one individual team so far ahead of the rest. So in my case at least, I'm better off just using every element.
So, I think that in the long run, you'd be better served to at least casually work on all of your weapon grids. That way, you don't end up in a situation where you roll some awesome stuff in another element and lament 'oh no, I never touched that element, I can't use this!'. With the recent gem gacha changes, that's not hard to do once you can maintain the 15k gems a day cost. So, in the short term, work on water first until the 25 ap gem quest daily is a non-issue. That takes care of... 12-13k or so gems each day. And the last couple thousand comes from just being active and fighting wherever.