Two-Headed Dragon has a lot of resistances and SPR, making it quite bulky. It's also able to reduce your ATK and MAG with its counter ability. It has little to no resistance towards Dark and Slash, and mostly attacks with magic.
If you have strong Dark slash attackers like Duane or Sterne (Knight of Ruin), then he should be somewhat easy to deal with, especially if you have a dedicated tank so they don't get hit.
If you don't have strong Dark slash attackers, bringing a unit that has a green mage job/subjob to use Imperil can help open up your options for units to bring. Otherwise, bring units with a Knight job to reduce 2HD's ATK+MAG and AP to make things easier.
Recommended Units
You'll have a pretty easy time if you have either Duane or Ruin Sterne, but any other Dark Slash damage dealer can work. Ideally, it's better to go with ones that can attack from a decent range to avoid 2HD's wide counter attack area damage.
Shadowlynx's job lvl 19 can be a pretty good debuff tool to increase your damage, so she's a decent option in any team.
King Mont specializes in area attack resistance since he gets them from master ability, TMR buff, and the free VC "King of the Lions, Mont Leonis". He's also able to reduce enemy's slash attack resistance. The only thing you need to do is increase his hate and move away from the team to avoid splash damage
Even though Whisper is a Dark element, she has a lot of defensive skills and some abilities that lower enemy stats. This can end up saving you 2 party slots since you have a tanker and a debuffer in 1 unit.
Rain is a magic tank that can also use King of the Lions VC for the area attack resistance. His skills are already pretty good against magic attackers, but he probably won't be able to contribute any damage due to 2HD's high resistance to fire and magic.
Green Mage main job for Imperil, and Time Mage sub job for Haste, Quicken, and Slow. A pretty good support against 2HD, but doesn't have any heals.