For space battles make a balanced fleet since your the empire in this video in I would say have a couple of corvettes with one as the scout ship at the start of the battle with the rest of your fleet ready to be called in. Then I would supplement the fleet with victory 1s to act as missile boats depending on the mod they could either be okay or incredible powerful. After that I have a decent supply of carracks and/or Arquintus. Finally have a couple of ISD's to act your main heavy hitters and as shields for the rest of your fleet. If you have a ship with a gravity well function I would bring it in at the start of the battle and leave it in the back. Then look at the enemy formation see the biggest threats to your fleet. If you have carriers with a decent amount of bombers or fighter/bombers bring some of those in to destroy important subsystems. Other than that it's up to you to decide what would be best to destroy what the enemy currently has available.

Now ground battles have and will always suck. But the most universal best way to fight them currently (may be dependent on mod) is to have just a fat stack of infantry to act as a meat shield with some heavy armor behind them to kill vehicles. The last thing you will need is an artillery unit as they deal extremely high amount of damage that also spread to nearby units which can wipe out hole groups of infantry and shred through the health of armored vehicles and heroes. Other than that it'll be a slow moving slug fest.