Bring Dunovs, Radiances and Kortuls against high level titans. EMP, Detonate Antimatter and Destruptive strike can all keep titan antimatter down and thus lock down their abilities. Discordias can also increase titan ability cooldown.
Frigates can really kill a titan a lot faster than titans/capitalships alone, but if you feel they're just feeding a titan start build carrier cruisers. They can stay away from the action while their strike craft do what they to whittle them down from afar.
Also Celio Command cruisers with designate target and Jarrusul Evacuators with nanites are great against any single target you need destroyed in a hurry, including titans. A handful of overseers, hoshikos, and even dominas are good if for healing if you can keep them far enough behind the titans guns.
Of course its the big fleets you're bringing in with the AoE units that are tricky. Your own powerful AoE attack would be ideal but not every faction has one. Obviously once you got the titan's antimatter under control its safe to bring your own fleet back in, but its admittingly a tricky situation.