The Hi-Lo game in the underground casino is rigged... in the player's favor. Specifically, the games follow a set pattern. That means that with trial, error, and the F12 key, players can detect the pattern and exhaust the deck by winning 27 hands in a row. That makes it possible to get the A and C endings, as well as every shop that the town shops have, before fighting one single battle. Considering how much the difficulty spikes after the 15th floor, having top-tier accessories and a few hundred Hi-Ethers are practically a necessity.
On an unrelated note: I can't figure out how to open the door in the right-side alleyway on the 10th floor. According to the Dou-Review walkthrough, there's a bonus boss behind it, but I cannot figure out where the key is.
[2024-05-05 Edit: I incorrectly stated that this method allows players to unlock Ending B before fighting a single battle. This is not possible because Ending B requires 10 of every type of sexual encounter, which the town and the tower exterior do not provide. On the bright side, grinding (in the non-sexual sense) on floors 9 and 23 for Perverted Gentlemen, floor 12 for Anal Masters, and floors 18 and upward for the Muscle Presser enemy variants will help unlock Ending B.
I regret my previous error.]