First off: the knack in becoming efficient in combat is to have an efficient Skill Deck. As combat is pretty hectic to start with (you need to walk, aim and 'fire' your Skills at the same time), you don't want to spend much time looking at your Combat Bar trying to figure out which Skill to use, especially as you have no standard attack outside of your Skills.
The key to efficiency is creating sequences.

You can modify your Skill Deck by dragging the icons from your Skill Book (B key) to your Skill Deck (N key). Moving them around within the Skill Deck swaps them. Drop them outside the Skill Deck to remove them and hold Shift while clicking and holding to duplicate a Skill icon. (Note that multiple instances of the same Skill all share the same cooldown at any given moment!)

A sequence is a column of Skills in your Skill Deck that have a certain synergy. Firing off one Skill will start a buff/debuff (positive/negative status effect) that the next Skill in the sequence can take advantage of. Having a proper sequence means that you can continuously attack, even when it reaches the last Skill and then falls back on the first in the sequence. That means you can keep the left mouse button pressed and only have to focus upon movement and aim.