8 appears to be a lot tougher than it used to be since the introduction of guardians. Specifically attacking a planet really has some bite behind it if you screw up. A strategy I often use is to send in a few raid starships (even the mark 1s will work for this) and let them shoot up the tachyon guardian if possible or just get out of dodge if not. Very often this will enrage the AI to the point of sending many of its guardians through the wormhole to my planet, where my turret defences plus full mobile fleet make short work of them. At this point you should be able to counterattack with your own fleet and take out their guard posts ASAP.
This has worked for me in multiplayer by myself where my ship caps are pretty pitiful compared to the computer's, so I think it should be able to work for you if you are careful with the timing to not allow the wave to occur when the guardians are also hitting you.