Aiming for the feet (when possible), zooming in or switching to the overhead tactical view can help.
For skills/spells you can use the turn order portraits at the top of the screen to target.
For normal attacks, right click an opponent and select Attack (no effect if you miss) or hold the Shift button down to queue commands and left click: if you get a red target selection indicator, release the Shift button, otherwise if you get the white move indicator, right click to cancel.
