For this game, I think mapping attack to left click would be totally fine.
When making keyboard controls, think about where a player's hands might normally rest. Most games have players hands at WASD and mouse. Depending on how frequently a player is expected to use an input, or how complex that input is, should influence which keys you use. High use, high complexity (meaning used at the same time as other keys) should be WASD or as close as possible (Q, E, R, 1, 2, 3, 4, etc).
When in doubt, playtest with others! :)