VC is obtainable after defeating the rare raid level 5.
Ranan is the only unit in the "Large" raid bonus so it's worth bringing him, even if he's not maxed. His damage is pretty good + he can increase SPR pierce rate and decrease enemy magic resist as well as an attack that can inflict confuse on Kraken.
Ibara is pretty much the strongest Lightning mage as of the raid's release. Even though her advantage of having some human killers isn't useful against Kraken, she can still hit very hard and has the ability to lower Kraken's Lightning resist to help your other Lightning units. With these 2 on the team, you'll likely be able to hit the damage limit pretty easily without even needing to chain. Though, there aren't any multi-hit abilities to capitalize on it.
Both Europa and Ramuh esper are pretty good boosters to any Lightning mage against Kraken. Both of them are very similar since they have Lightning and Magic ATK increase + Water killer and Aquatic Killer, but not enough SP to take them all. The only difference here is if you rolled and get Europa, you'll also get a "Large" raid bonus just for owning it.
These 3 are other good Lightning mages if you don't have Ranan or need fillers.
Skahal is "Medium" raid bonus unit. Has Lightning Res- at job level 25, but can also just be used as a time-mage support for Quicken.
Sylma has Jamming Thrust and more spells from Kotodama Wielder sub job. Resistance Break to decrease SPR at job level 12 too.
Salire at job level 25 can reduce single target resistance. After that, it's just Thundaga/Thunder spam.
Aigaigon and Coeurl have nodes for Lightning ATK and Water killer, but Coeurl also has Aquatic killer.
Other Lightning units, if they aren't a magic damage dealer, are kind of difficult to recommend since Kraken has high resistances to other attack types and a good amount of DEF. If you only have good Lightning supports, Kraken's Strike resist is the second lowest, so Charlotte, Velne, and/or Meriluke for their Monk jobs to chain strike is potentially doable.
Other elements with good mages are definitely viable too, which means you can go with the ones you have the most support on.
2nd best element of choice is Wind, mostly because 2 of them are in the raid's bonus units. This might be less accessible to a lot of players since they have more limited-time units, but it's a decent reward boost if you happen to have them. There's also Gargas, Moore the Merrier, or Varush to complete the team.
Dark is an easy recommendation if you have Helena (Black-Robed Witch) since she can reduce SPR and Dark resist by herself, but also because Dark has other good options like Garvall, Golbez, and Lemure to make a full mage team.