Maybe watch some VODs on Twitch. I used to watch some streams by an account that I think was named just "Shattered Pixel Dungeon", and he was very good about it explaining his thought process as he played. I learned a lot of advanced tricks.

But he tended to talk about more of the super advanced tactics with challenges enabled more than the basics. Just watching and paying attention to a stream can help with the basics, though. Especially if the streamer is live and you can ask questions.

Surprise attacks can be performed in many ways, and using them for guaranteed hits is an excellent strategy. You know those tiny rooms with a column in the middle that's only 1 by 1 tile? Those are surprise attack factories. You can also get enemies to stop following that way if you're in serious trouble. I'll discuss both of these things below.

Here's how movement and surprise attacks work. You move, then all enemies move (or attack). The enemies only seem move the instant you move, but there is a moment of time between your movement and theirs. If, during that moment, they can't see you, then your next attack will be a surprise attack.

So, here's what you do:

1. Let an enemy get near you while you're next to one of those single-tile columns. (Orthogonally agacent to the column, not diagonally. The enemy must also orthogonal to the column and diagonal from you.)
2. Step to the side of the column away from the enemy and it will follow you around the column.
3. Attack.

Since you moved first, for a split second the enemy couldn't see you because you were behind the column, so your attack counts as a surprise attack and can't miss.

Instead of attacking, if you just walk around the column another couple of times, you will briefly see the "?" thought bubble appear above the enemy. At that point they've lost track of you and won't follow you. Next turn they will walk in a random direction, so you can now walk directly away from the column to get some distance. Sometimes you get lucky and they walk the other way and don't immediately see you again. You can also just wait behind the column and see which way they go and repeat this trick until you get lucky and they walk directly away.

I use the above trick sometimes to get away from enemies when I am very near death and I can hope to kill that enemy with ranged weapons or something, or when I accidentally aggro an animated statue by mis-tapping the search icon when trying to inspect the statue to see what his weapon is, and I end up walking right up to it and punching it. 😅

More often I just use the guaranteed hits to kill the enemy while taking less damage. Of course, you have to balance this with food. You don't want to kite every enemy to one of these columns because you'll use up too much food in the process.

Doors work the same way, as I'm sure you know. When you walk through a door, you close it behind you, and in that instant the enemy on your tail can't see you, so when they open it, you get a surprise attack. In some cases, I'll walk through 2 or 3 doors when fighting the same enemy to get the guaranteed hits. Sometimes this backfires, though, when you end up in a narrow hallway with a second enemy in the way.

There are dozens of little tricks like this in the game that make it beatable. You mostly learn them by playing and experimenting, but you can also learn them by watching streamers.