The fight starts with 4 Ogres with Ramuh, and all of them are Lightning element. The Ogres uses physical strike attacks, but Ramuh uses mostly magic. After 2 of the Ogres are defeated, 3 Lightning elementals will show up at the bottom corners of the map.
The map's unit placement has 1 spot that is close to Ramuh, and also in between the 4 Ogres. This can be a good spot for your dedicated tank to soak up the damage. You can either turn on the "initial placement" option before starting the fight if you want to manually select their position, or put your tank in slot 2 of the pt.
Take note of where the Lightning elementals spawn. If you don't have a dedicated magic damage tank, you can potentially burst them down early before they spam you with area lightning attacks.
If you have trouble surviving, have your tank lure the enemies to any of the corners of the map. Make sure not to group together as well since Ramuh and Lightning Elementals uses area attacks often.
Tank units can really help with the fight, but may not be that necessary. You can bring either a physical tank, or a magic tank, or bring both and alternate the hate increase if your account isn't fully developed yet.
Since there are 4 Ogres at the start of the fight, having a physical tank at the start and place them in between the Ogres can mitigate a lot of early turns damage. Once the Lightning elementals shows up, increase your magic tank's hate and don't group up to avoid area damage from the thundaras.
King Mont alone as the tank can be sufficient enough, despite being weak to strike against 4 Ogres. He can be built tanky enough to survive the attacks (especially if you have Encounter of Heroes), and he's not completely defenseless against magic attacks since Mercy's Grace, or use magic resist setups with Platinum Helm and/or Fenrir VC. Ramuh can also be stunned, so Mont can just keep spamming Stun Blade and hope to get a free turn.
Similarly, winter Vinera can also be the main tank for the fight. Give her DEF gears to survive the early turns, and use her spellblade skills to tank the magic attacks later on.
MR Mont is a physical tank with an EX. Killer Drain Blade has human killers to help against the Ogres, but it may be a struggle to get AP to maintain hate and deal damage. If you have Illusory Bell you can use Taunting Blade to get hate, and Breakdown on Ramuh to survive.
In terms of damage, as long as you're not using Lightning or Water element, most units can do just fine in this fight.
Yuna is a pretty good support since she can heal, revive, and deal damage. Elsirelle simply adds to the Light element, and also useful to help clear the "Elemental Chain of 3 or more" mission. You can add in Sakura to get more benefits through all the Light units' master ability and to be the damage dealer.
There are limited options when it comes to Earth damage dealers, but Louelle is a good unit to help burst down Ramuh. You just need to make sure she doesn't get targeted often, which can be somewhat doable if you have Vow of Love VC even without a strong tank.