-generally- I take them down by throwing fleets at them. Usually turns out better than throwing starships at them.
Since K costs generally went down, I find that at 7 AI difficulty, marks I-IV of fighters, bombers, and missile frigates can clear a 'popped' DG lair and the three guardians that spawn. MKs I-II Fighters, Bombers, Missile Frigates can struggle against a single dire guardian, it depends, but a bit of extra support will help a lot (as long as that support doesn't go in front and get alpha'd, or have the wrong bonus hull type).
Fortresses will die horribly to guardians that get close enough, but they can put a significant dent in if far enough away and screened by fleetships. Golems should also operate from behind a protective screen in most cases. I would recommend you try Spirecraft Implosion Artillery. They can outrange Dire Artillery Guardians. You might need more than MKI Implosion Artillery, but if you have the room, you probably have the speed to kite. Alternatively, perhaps you can use cloaked Rams.