That's a pretty good score already. The tower requires some very specific girls and, unless you have massive resources, quite a lot of trial and error and some luck.
I usually don't spend money on these kinds of games, but I have bought 5000 point gachas twice for the rainbow exchange tickets. One I used to get Tarragon and the other I don't even remember what I used it for. Not that I really needed to, but I don't mind rewarding people for their work and the game has been pretty generous with freebies and whatnot. Other than that, I feel like the game is absolutely playable without spending any money.
That said, I don't think I would have been able to complete the tower without single-mindedly working towards it for the past several months and pouring all my resources into it. Rarity growing 3 girls to beat just a few floors alone is proof of that.
I can't one shot it either, but something like this should work with a fair bit of luck.
Three girls to tank/evade the damage with taunt and two to boost the damage to kill one of the pests as fast as possible. 4head can actually survive in this setup for a while thanks to her guts and does great damage to the pests with pierce/yellow weakness.