u got 2 choices
1- focus on light, meaning start leveling and raising weapon skill for light weapons. if nutaku follow same event schedule, next event has great light eidolon and weapon to start with, and pull gacha there. i think problem with light is not enough guaranteed or easy obtain Kamihime, not eidolon or weapon.
2- undecided yet, instead of raising weapon skill on light weapon, just level raw stats on everything and see what happen in future.
but realistically, that is more of long term goal and whether u choose 1 or 2 u still going to be doing lot of the other for now to get stronger. so i would not worry and just keep raising everything. example, if u have extra R weapon u can use it to skill ur bows now, no need to stock it for different element.
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elaboration- with sol and artemis u have heals and burst. if u work towards good balance for team. my advise is aim for Mordred for ur first Legendary Soul. Mordred ability raise chance of debuff application, and then first ability applies ATK/DEF debuff and lot of DOT. combined with Sol ATK debuff which stack it, and then Black Propaganda which make boss ougi less often, ur team will take much lesser damage from hard bosses, and Sol can heal for a lot and remove boss buff.
if u decide to switch elements, Mordred can switch to new element with different weapon, and the combination with Sol is still effective. It does not matter if Sol does not get benefit from new element because her usefulness is atk debuff, heal and dispel boss, not necessary damage. other party member can do damage.
if u decide to play light for now, u could farm Light Prison for the SR sword for Mordred to use as mainhand for light element, although how long before it drop is up to ur luck. or just wait for light event.