Sol isn't really that hard of a boss once you figure out what his deal is, but walking into him unprepared can be a real shock.

The second time you fight him, the game challenges you to beat him in six turns. On top of that, the first time you fight him is after a challenging boss fight with Dark Fina in the same level, so you have to fight him partially exhausted.

The fight takes forever, even if you can manage to keep his defense broken. Veritas of the Earth buffs his defense, debuffs your offense, and hits pretty hard on top of that.And finally, his first appearance is the only major boss battle in the game where you're not given an option to have a friend's character carry you through the battle if you're not strong enough. and if you gave your tank the Golem esper (as its stat buffs are ideal for tanking), they'll be particularly vulnerable to his attacks. On top of that, he has many air-element attacks (which are very difficult to defend against unless you've opened the right chests).

Veritas of the Heavens loves to spam area-of-effect attacks, has ridiculously strong defense, and is one of the first foes that's completely immune to all status ailments.

Final fantasy brave exvius earth shrine