i've already decided to main light a long while ago lol (this thread is old)
as for fire... i still have that miracle ticket.....
tends to be the case for stuff like thunder builds that have no healer
never, ever do this unless you're a new player and have no better eidolons than the phantom lilim (even then, you probably should have rerolled)
only exception is if you need a healer or specific debuff... or someone to absorb damage...
as for setting up a grid (if that's what you meant by setting for the weapon), you'll want the following in this order depending on what you have:
-MAKE SURE WEAPONS MATCH THE ELEMENT OF THE TEAM YOU'RE MAKING-
1) Weapons with 2 skill effects (all SSR as far as I know)
2) SSR weapons with Assault
3) 1 or 2 SSR weapons with Defender or Pride (depends on what else you have, but only have 1 defender and 1 Pride weapon total at most)
4) SR weapons with Assault to fill the remainder
5) If you don't have anything else remaining, toss on SSR or SR weapons that have Defender (or extra pride weapons if you have them)
6) if you're out of the rest, get the weapons with the best stats to fill in the last few slots
so for instance, say you have a fire team and you have 5 fire SSR (1 pride, 2 defender, 1 assault, 1 double effect), 3 fire SR (2 assault, 1 defender), a lv125 water SSR, a lv40 wind SSR, and a lv85 light SR... your grid should use the following
-all 5 fire SSR (hopefully one matches what the soul can use)
-all 3 fire SR (again, try to match a weapon to get your soul fire element if the SSR don't meet this)
-the lv125 water SSR (very high stats at lv125)
-the light SR (lv85 SR should have higher stats than the lv40 SSR... some maxed SR even beat lv75 SSR stats)
that's just an example of how builds can work. but if you're trying to eventually fill spots and can't get any SSR to do so, grind out raid bosses (the disaster raids, not the catastrophe unless you can live long enough for others to help you kill it). raid bosses can drop weapons, 2 of which are defender but you want the 1 assault weapon they drop.
as for setting up Eidolons, i tend to pick ones with buffs and debuffs i could use (double attack, attack, elemental effects that help for the fight, defense), while going for the highest stats. as the main one, it depends on what's the best one for you to use. for instance, it's better to use jabberwocky over the wind lilim for a wind team (50% atk and a slight ability damage boost to everyone as opposed to cutting in 2 off-element members to try and get the atk boost).
that's a lot to follow, but you'll get the hang of it eventually. as for what kamihime to use, that's honestly not my department when it comes to help, but some clearly are better than others. obvious kamihime that are worth using is Svarog, Mars, Sol, Gaia, Titania, Moonligh Maiden Tsukiyomi, there's quite a list (even some SR like Diana and even some R like konohana-sakuya are worth it). This might change way later down the line though (and you'll want every scene to get all the magic jewels and abilities) so you'll honestly want to work on every kamihime you get your hands on... at the very least, lv25 for R, lv55 for SR, and lv65 for SSR. try to max them out if you can though, but focus on the ones you'll use first
souls is the same thing but the go to soul for most cases seems to be mordred... cassiopeia/andromeda for if you need healing, joan for if you need to defend against strong attacks (this can save your run in a lot of harder content), it depends on what kamihime you have to fill the slots (for instance, you probably don't need Joan if you're using Gaia, but having both can let you survive a LONG time). Souls are more slot fillers to the kamihime you have but still make or break your team just as much...
that's all i got for this. it's a wall of text but take your time reading it and you should pick up on a lot of this eventually