A friend's game involves the players fighting a monster that roams the city. This monster is controlled by a deck of cards that move it, and cause it to attack. Each turn, the players draw a card, and follow its instructions.
You might imagine all kinds of cool things to put on these cards. The monster might move different combinations of two directions. It might change direction when it hits a wall. It might move, then also destroy stuff or attack. None of this is necessary. This is all just complication and work for the players, and it adds nothing to the gameplay.
The monster just needs to function properly, so the players can combat it. It can be a rampaging and mindless Godzilla. It doesn't need to be some super-strategic ninja, whose antics occupy half the game time.
The correct answer here is the simplest one. The cards should just say "The monster moves one space west" or "Everyone on or adjacent the monster loses 2 health."
You then design the rest of the game to work with this simplicity, rather than using complexity to solve problems. If the monster isn't doing enough damage, because it's not attacking enough, you increase its damage. If you're annoyed that it hits the edge of the map, you just add an occasional "teleport back to the middle" card, so that wall-hitting is rare.
A bad designer simply lets out their belt, and adds complexity.
The rulebooks for my games tend to be about one or two pages long. This is because I avoid creating rules. People are surprised by how simple the rules of my games are.
Simple rules let players start playing immediately. They can devote their intellect to formulating a strategy, rather than to wrapping their head around the rules. It's just a much nicer experience to play something simple.
Avoid creating rules for certain conditions and times.
Imagine you're creating a dungeon-exploring game. What if there are no monsters left in the dungeon? You might create a rule that adds a monster. What if too many monsters end up on one space? You might create a rule that separates them, or a rule that says they can't go onto the same space. A great designer deals with these problems without rules. They construct the game in such a way that these problems never happen. I can't tell you how to do this, only that you must do it.