There are already two dps-tank jobs, please don't make Paladin the third.

[Violette]

Except that, as established, boss auto-attacks can consume Sheltron and you're SOL at that point. Timed blocks work in games that require precise gameplay (see: Dark Souls, Dragon's Dogma, the Soul Calibur games). FFXIV is far from that and a different approach to that ability should be considered.

I'd be willing to start by reducing the duration of Sheltron to 5s and making it block all incoming attacks and see how that goes. This way you can go for that illusion of timed blocking while not making the ability overpowered nor making it easy for it to go to waste.

Thing is, as we can see with Heavenly Heel, it's wonderful against PHYSICAL tank busters with a cast.
It'd eat Flatten for breakfast. It's more victim of circumstance than a bad ability.