Weapons are a case of farming the living hell out of raid battles (disasters/ catastrophes) and getting their assault drops (out of gold and plat chests in normal/ expert and gold chests in ragna). You should at least be able to solo the normal ones, which have kinda a chance to drop the weapon from the plat chest (1 for spawn, 1 for MVP) although with a lower gold chest drop rate, though that doesn't matter as much. Since getting a full grid of weapons is an eventuality, MLBing those is the better option since you'll be wasting enhance materials otherwise.
Eidolons for newbies... are tricky. Patience to get a full grid of 6 SSR eidolons is a factor, the other is the cost of MLBing them, which often to new players looks pretty intimidating. Assuming you're at that stage, it might be better to partially LB one and just put the rest as sub. Do not LB 2 separate eidos, if you enhance a 2* eido with a 1* eido, you will not get 2 limit breaks from that.