What people have already said will pretty much work. But I'll try to be more specific about its moves.
The Kultorask has three moves to worry about (the fourth just heals its own fleet):
- Nano Leech, the ability that heals the Kultorask (and restores antimatter), affects frigate and capital ships: "frigates" means any ship from the 'build frigate' and 'build cruiser' menus, except for corvettes.
- Dissever does damage to frigates, capital ships and corvettes frigates and capital ships - and it does more damage used in conjunction with Nano Leech (but only to cap ships and frigates). This means it has a good chance to kill corvettes instantly, but it still can't heal from them. Dissever is its level 6 ability, so no worries if it's below level 6.
- Finally, Gravity Pulse slows movement and attack speed for frigates only. It can also disable frigates which are affected by Nano Leech.
This means most conventional fleets are generally...bad ideas:
- No frigates at all - they will only ever help the Kultorask. That means no cruisers or corvettes either. Carriers could be considered, but should probably be ready to jump away if the Kultorask gets near them.
- Corvettes certainly wouldn't help the Kultorask, so they could be used in great number. If the Kultorask is above level 6, however, Dissever can kill them instantly, and they may just be a waste of resources in that case.
- Since Nano Leech+Dissever also affect capital ships, using a lot of caps is out too. Using one or two at a very high level would probably be beneficial, however.
The only three offensive unit types the Kultorask's abilities can't affect are tactical structures, titans, and starbases. So, to consider what to send against it:
- Both TEC titans should defeat a Kultorask easily.
- Given the Kultorask's relatively low dps (without abilities), an upgraded Orkulus should make short work of it.
- I'm not too sure about Advent titans, but a Transcencia should work too. Fill it with squadrons+upgrade its health. The Kultorask's only winning move would be to leave the grav well.
- If you're at your own planet, however many turrets and hangars you can get away with would be good too. Low dps, again, means the Kultorask can't destroy them too fast.
Personally, while it might currently be OP, I'd like to see every titan have something which scales with fleet size, enemy or ally, with no cap. It'd make them more than just big cap ships, so even in a 2000vs2000 game they're a central point of the battle. It'd also mean, as with the Kultorask, taking down each titan would be a less orthodox affair.