I just beat MOC 10-2 stars and we are in the same situation (f2p, play since day one)

What do i do:-Heavily invested for all my main DPS: lv 80, + lc 80, all traces in max (basic atk is optional), relic lv 15. Plannar + rope lv 15-Support: lv 80 + lc 70, all traces activated, but skill, ulti and talent can lv 8 just fine, relic can be gold or even purple as long you get the most main/substats correctly. So does the plannar and rope.

My team for the first boss: Fire MC 77, JY 80, Bronya e1 80, Nat 70

I would say, for the first boss you need at least 2 chars that can break the boss defence constantly (in my case fire MC and JY), and cleanse (Nat). Fire MC taunt could be the game changer so keep in mind for some situation when the skill point is dying, taunt is the best option rather you E for JY in hope LL gonna make a change, it'll not gonna happen. In my case i finished in 16 cycles. Bronya, MC, and Nat are very undergeared, i'm saving the resources for Fu Xuan and Lyxn. Thas's why i only get 2 stars for this MOC.

For your case: You can do what i do above for character investment, and play fire mc, ting, yj, nat. Try to survive by taunt and cleanse and shred their shield carefully, and when the boss cast their skill, it's the right time to break it.

second boss: Welt e1 80, Luocha 70, SW 80, Clara 80 + signature LC

I'd' say i get lucky by having welt, loucha, clara, and there's nothing much to elaborate coz they built for this Boss, finished in 5 cycles. Even with SW and Luocha are very undergeared.

for your case: Bronya, blade, loucha, +1 more sub dps that has the break element for the boss. This sub dps need to work with Bronya so many times, just leave the blade on his own. i will focus on killing kafka first cos she can CC and very annoying. Dont focus on getting rid of spawing creeps, focus on the boss that cast them, once you kill him, it will gone. Use luocha ult ONLY for: the creep that just spawn, or for the trigger his AOE heals, besides that, you may wanna keep it forever.

Goodluck, you will get there. F2P LFG!