Red Giant is weak to Magic and Dark, so Garvall and Helena are definitely the easy and ideal choice. Together with Diabolos VC for dark damage increase, and Diabolos esper for demon/light killer, these 2 can potentially reach the damage cap with each attack, even against level 100 Red Giant.
Vinera can reduce DEF, while EX Shadownlynx have single target attack reduction. These are pretty good supporting ability that can increase the damage of your other allies' attacks.
Knight of Ruin Sterne can deal high damage because he has 2 Light killer abilities, making him a pretty strong carry. Recommended to put Illusory Bell if you use him.
Since Red Giant doesn't have any elemental resistances, most Slash or Magic type damage dealers are viable. What will help are the setups that you bring against it. It's recommended to focus on a certain damage type instead of a mix of Magic and Physical to get the most benefit out of a VC party ability.
Ifrit is good for Fire units since it has Fire attack, and Demon killer ability boards.
Demon Wall has Light and Slash resist and some Demon killers, but if you want to take Demon killers, then you can only go with either a Light or Slash resist, instead of both.
Typhon has Wind attack up, and Demon killers.
Iron Giant VC is pretty handy and a convenient defensive + offensive option to use.
EX Vinera and Shadownlynx might not be the main carry of the team, but Vinera can reduce enemy's DEF by a lot and has some good sustain abilities, while Shadowlynx can reduce single target resistance to help your other units to deal damage.
They are also part of the raid event's boost to make the farming easier. Main issue is they need to be at high EX job levels to get those skills, which might not be viable to a lot of players.
Even if you don't EX Frederika, it's still worth bringing her against Chocolate Flan for the unit bonus. It's weak to Missile and Lightning, so she's able to deal high damage to the boss, especially if you also have Coeurl / Tetra Sylphid esper, both of them have Missile attack increase and Aquatic killers. At high boss levels, you'll need more than just Frederika to deal damage, but at lower levels, the other unit(s) can be anyone like a tank or a support if you have Frederika with Illusory Bell.