Genshin Impact

This guide is mainly intended for F2P players like me.

You only need to clear Floor 12 for this trophy; hell you don't even need to get any stars at all (I didn't lol). So long as you finish within the time limit for each chamber you'll get the trophy.
You do however need to earn at least 6 stars each in Floors 9-11 to unlock the next respective floor. Floors 9-12 will reset every two weeks though, so you only have a limited time to do it before your progress resets back to the start of Floor 9.

Needless to say you'll need to have invested a great deal into two full teams of characters to even attempt a clear of Floor 12 and it's difficult to recommend team comps as unless you whale hard you are at the mercy of the gacha gods for characters. Theoretically, any character can be specced to tackle the Abyss solo, though this is obviously incredibly unrealistic for most players as different characters have different intended roles as demonstrated by their abilities, constellations talents and stat growths.

The bare minimum would be to invest the most into extensively upgrading two dedicated damage dealers and build a team around enabling them via support DPSs to help with elemental reactions and healers/shielders. Mine were Yanfei and Kamisato Ayaka, though again you will almost certainly have different ones. If you are looking to target specific helpful characters then I would highly recommend Diona, Xingqiu and Bennett. Diona is one of the best healers and shielders in the game and Bennett is great to heal and boost your attack. Xingqiu isn't quite as mindless as these two but his Raincutter Burst was fantastic for both my damage dealers (though especially Ayaka in my freeze comp).

The most important investment is artifacts and the grind for them will prove frustrating depending on your character and it is far too complicated to go into here as again, you will almost certainly have different characters than I did. I would say that only the main damage dealers need heavy investment. Main stats for the Goblet, Sands and Circlet are the most important when farming sets, though if you can then for your main damage dealers prioritise Crit Rate/Damage and Attack% with at least one elemental damage boost depending on who they are (elemental damage boost can only be a main stat). This can prove annoying as the game loves to prioritise HP and Defence for artifact substats, so if you get one with at least two or three of these then prioritise that. You're not looking to 36 star the Abyss after all so even a couple will be handy, especially if you already have a good main stat. They also don't really even need to be 5 star artifacts either; my Ayaka had a couple of 4 stars on her.
My Yanfei was at Level 90 with a 4 Wanderer's Troupe set prioritising Pyro Damage%, Attack% and Crit Damage and my Ayaka was a Level 85 with a 4 Blizzard Strayer set prioritising Crit and Cryo Damage. I would only recommend upgrading them to Level 80 and ascending them a final time, as 81-90 has very limited stat boosts. For your support characters I would recommend levelling them enough to unlock their final talent (Level 60-70 usually), although with Diona I fully ascended her (really prioritise Diona if you can she is a fantastic character).

Needless to say in between the artifact and material farming and inevitable F2P wall you'll hit this will not be a quick process and will take the average player a good few months to get from the start to being able to tackle this, but I just wanted to show that it is possible without spending a cent. There are so many guides out there for the game that should be able to help you depending on your available characters. My one piece of advice is don't be wasteful with Fragile Resin like I was. As a F2P I would say avoid Condensed Resin entirely and only use Fragile Resin to boost your stocks if it's running low.

Good luck and hopefully bearable grinding.