With regard to Sealion - and Magicites in general - I find that selecting one damage type (either PHY or MAG) is better than mixing. That's because, generally, you want some kind of Boostga/Faithga, and a mixed option (e.g. OnionBurst) won't give you as significant a boost as a single option (e.g. Shout). As it happens, I have OK's pUSB and mUSB, so he can cover either base, but it's something to keep in mind where possible.
In terms of how challenging Sealion is, he has a mix of PHY and MAG, which can be tricky to defend against. He also has a Water attack (which unlocks at low HP) in addition to his Ice attacks, so there's no easy way to put together your resistances. You may need two healers for this guy. Plus side is that there are a lot of options for Fire damage, including Meltdown, Valigarmanda, and Smoldering Fire.
Personally, I brought a MAG team to Sealion, as follows:
- Papalymo: Firaja, Chain Firaga. Enochian Firaja. Thaumaturgic Flair, Overseer of the Circle, Forbidden Arts II.
- Onion Knight: Meltdown, Protectga. Forbidden Wisdom. Sorceror Sublime, Swordsman Sublime, Dr. Mog's Teachings.
- Terra: Chain Firaga, Chain Firaga. Omen, Inferno Combo. Knight's Charge.
- Edward: Angelsong, Mage's Hymn. Whisperweed Ballad. Zeal.
- Relm: Curada, Shellga. Divine Portrait. Mako Might.
Admittedly, this list is out of date. I could replace Edward with Ulala, for example, since her LMR occasionally tacks medica onto BRDs. I could also use Terra's Brave USB, perhaps. Or LD her.
The good news is that this is not a battle requiring you to consider status ailments. My gear is instead designed around Water resist and otherwise stat numbers.
Edward is mostly filler, but his function as a force multiplier is powerful. Again, you can substitute any BRD for him. Angelsong helps substantially with off-healing, while Mage's Hymn gives a stacking powerup to casters. (If you're using a PHY team, there's a PHY equivalent.) I think it's worth honing, at least for Magicites.
Papalymo with his BSB and an LD is extremely powerful for this fight, and generally. With a full dive, he gets +Fire and a moderate chance to W-cast. With the right RM, that becomes a chance to triplecast. His BSB boosts his Fire, his CMD2 boosts his MAG, and between that, OK's pUSB, and Mage's Hymn, his CMD1 is absolutely unloading damage on Sealion.
I bring OK in part because of his mUSB, and in part because of his versatility. True, there are other sources of Sheepsong, but OK can use a BLM (e.g. Meltdown) to deal damage and charge gauge, and he can bring half of the ProShellga duo, which is important in a mixed-damage fight like this one. Relm brings the other half, along with Divine Portrait for emergency healing and well-timed non-charge.
I can say that, when I first tried him, he was super hard. But once I had this team assembled - and various iterations of this had been used successfully on other Magicites, so I was already familiar with the mechanics - he was much easier. He's one of few that I can consistently sub30. So part of it is your team, and part of it is simply experience and familiarity.