Short tips for attacking
When it comes to Ground Combat, remember only four things that win planets.
1.)Having a sufficent military force to overcome the defenders.
2.)Choosing the right attack.
3.)Have a sufficent navy to destroy any ships attempting to relieve your siege on your choosen planet.
4.)Never, ever leave your transports undefended.
#1 means exactly what it means. Overcome the enemy no matter what. Be it a few divisions to armies, always make sure that you can overcome the enemy through superior strength and firepower.
Note: Always wait for your Ground Units to be, from "Recruit" to "Trained".
For #2 in Choosing the Right attack, keep in mind what race you're going up against and their picks. Exploit their weaknesses and overcome their strenghts. If the race you're going up is a slow thinker (Cunning: Slow) use the Suprise Attack or Feint.
Also keep in mind the number of population left in the planet. Never ever use the NBC(Nuclear Biological and Chemical) Weapons if the population is low. Use them only when going against a superior force and a high population.
A little tip: I prefer to capture a few planets of Grendel and use them as Ground Forces. Do not how ever, utilize their support. Their initiative sucks. Instead use your won support units, unless your own iniative sucks don't bother at all.
#3 pretty much explains it all. The purpose of this is that the enemy cannot reinforce the defenders by landing additional units. if you can manage that, you've doomed the defenders.
#4 is the number one thing to keep in mind. Do not ever let your transports go attack a planet undefended. Be it a detachment to a flotilla, always have a guard around so that you can distract the enemy while you retreat your transports. (Retreat only if the guard faces a superior force.)