Micro is almost as important as are the strategy and the adaptation. You can't win a game versus a good opponent, without good micro. For every situation it's different. Depending on the unit mixes and races you have to move your units in different formations.

Example1 : If you are using bear/dry(dh+naga) in NE mirror and his bears are already in line and a few of your bears are after the first ones he will start with advantage over you because he will have more bears fighting, while half of your bears are still behind the first few bears. Before a battle you have to re-arrange your units in the best positions of your whole army(you can't use dry first and then bears right?)

Example2: If you are using a mix of hunts/archer/dott/dry versus orc and put all on attack move, and your hunts are in the middle of all your units you will lose time moving your units in the same battle line of range(hunts, dry, dott, archer+range upgrade) meanwhile your opponent will already be doing his micro in focusing unarmored units with demoilshers etc.

Before every battle re-arrange your units in the best battle lines, this will be most important before the battle itself. You may have more population, but if you don't use it in the best possible way, the advantage you have in population may be lost. In all different MUs and strategies you have to study what is best to do first.

Example3: When a Far Seer uses chain lighting on one of the units, don't hurry to use the scroll, better move the most damaged units behind those with more hp and wait for the shockwave. This way you will heal more damage with one scroll instead of using two.

In every particular case you have to train the situations many times to make it perfect and believe me - you can always perform better, not only in battles, but in your overall games as well.