The trouble with you guys is you assume you'll have little to no difficulty so your plans all involve just doing whatever the hell you want.
What I'm picturing (although I have no idea whether such a situation could arise in spore) is one race having a bigger, better fleet, and the other race being able to use the energy of a planet's atmosphere as a weapon. So you have this asymmetrical dynamic where the one side will be dominant in space battles, but if they try to just line up their cruisers and attack a planet, they would take horrendous losses.
What's the solution? Well they use a **SPACE** adaptation of
Infiltration tactics. They send some fusion bombs through hyperspace and detonate them in the upper atmosphere to cause a disruption (the enemy's technology is immune to EMP, but the atmosphere control isn't). At the same time they drop one or more troop ships out of hyperspace and unload commandoes in stealth suits. The commandoes deploy parachutes at as low an altitude as possible and reach ground. They proceed to disrupt communications, and more importantly, to sabotage the atmosphere control stations. Once this is done, the full invasion can start. They send in the common soldiers with mechs and so forth, going across the whole planet, the commandoes sneaking ahead and wrecking the AC stations, and the fleet bombarding in support of the ground advance wherever possible (IE, safe).
