it depends on many things. firstly you need to make your team dual element or better to be mono element. The weapon should be the highest grade because weapon with SSR give you additional 12 lvl skill either it is defender or assault. you need to aim for assault since it is better but defender also not bad. every level on the weapon gave you 0,5%. So SSR level will give you 6,5% increase.
SR only give 6 lvl additional skill level where R gave you zero additional skill level. If you somehow have 9 SSR same weapon. it is better for the starter to put them all in the weapon list instead of limit breaking it because the skill level will gibe you tremendous boost. when the time past and you need to limit break you can do that after you have enough resources.
Still it is all depends on your strategy. For eidolon it will be better to pick only SSR because upgrading SR with limit break most of the time is useless and still way below SSR with no limit break
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Saying it is useless is not a correct word because it will serve as your temporary additional firepower that will win you reward. What i mean being dual element or mono element mean it needs to match all your resource from eidolons, kamihimes and weapons.
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All strategy are case by case it depends on your situation. if you have abundant resource you can just M/LB them all since M/LB them give you extended limit and will increase burst skill damage.