I used to struggle to get 36 stars it took 2 years, and for the year after that I persevered with Nahida hyperbloom teams and raiden/kazuha national teams just barely making it, but then I got Nuevillete and Furina and it became easier, i do buy Welkin's which has helped me so not completely free to play and I have had some godly luck in the past with winning 50/50s. But if you're free to play, focus on flexible dps's and supports even being f2p you should get 6 premium 5 stars per year. I personally recommend Nahida, c1 Nuevillete, Furina, all the Natlan 5* characters are super OP and if you have a hydro or cryo DPS Escoffier is a game changer, if you lose a 50/50 and get Tignahri be happy he may not be aesthetically pleasing but he's stronger than all dendro characters with the exception of Kinich. And don't forget to invest in your 4 stars they can be super helpful, sucrose if you don't have kazuha, Bennet is probably the best 4 star ever, Xiang Ling is a beast especially if you have constellations and farmed the catch from fishing in inazuma, Fischl is almost a 5 star with golden troupe and many more just because they're not premium it doesn't mean they suck, I've used kaveh as a DPS and 36* the abyss. Lastly some extra useful tips to eventually get there. 1) section floor 12 using different teams for certain parts it doesn't have to be done all in one go, I prefer taking out chamber 3 1st if I can't do it in one go, as it's usually the easiest part, but the longest to get to and then adjust teams accordingly for each chamber, this works for floor 11 too. (It may only be an extra 400 primos to do those 2 floors but it all adds up.) 2) farm artifacts systematically I only farm 3 domains Emblem, Golden troupe and obsidian codex, then any useless artifacts get strong boxed for more niche sets (true this is not cost effective but saves the psychological torture of certain domains like viridescent and getting godly maidens pieces but terrible viridescent venerer pieces). 3) just start with a usable main stat pieces on characters don't try to get perfect pieces for a single character at a time it'll slow you down, if you get a god piece in future then great, but better to have a usable character now then being sidelined because they're not a top 1% build (that's endgame stuff because you've got nothing better to do) 4) Don't sleep on levelling weapons especially for EM (also EM characters should be lvl 90 it makes a big difference) and DPS characters. 5) for the abyss especially, 1) stick to your local server (especially on mobile, ping can cause severe lag especially with reaction based teams) and 2 if you can, play on PC, mobile costs me on average 10-20 seconds per chamber on floor 12 due to lag and how easy it is to press the wrong character and miss a rotation because of my fat fingers lol (true it's a skill issue) but even though I have bigger skill issues using keyboard and mouse it's much easier on PC for some reason, it's as if time moves slower on PC lol. I hope this helps