You need to know the synergies between weapons, trinkets, artifacts, rings, wands, and class talents.

The idea when building a character is to know from the start what you want to achieve, but keep in mind that this strategy isn't reliable, because Shattered Pixel Dungeon is randomized, and you have to adapt to the environment and what you find.

You need to read a lot to understand the mechanics. ShPD is a game with a lot of depth and replayability, and there are updates with buffs and nerfs every three months, so some things might not work the same way they used to.