King Mont is a cost 100 strong Fire tank, mainly for physical, but also has strong offensive attacks that come with relevant debuffs to be futureproof.
His attack-type resistances, passive, and stats are already pretty good, but his abilities also help make him cover a variety of different attacks. Master ability + TMR buff for high area attack resistance, Mercy's Grace for 15% all elemental resist, and Double Resist for +25% pierce and missile resist. Odin VC can help with the missing unit attack resist, but there's also his own King of the Lions, Mont Leonis VC for some more additional stats.
His Archlord main job has several great things going for it. He can increase hate through the passive (EX), and the self-buff Proclamation. The offensive skills have good effects on them, and he can easily deal high damage since the farmable weapon, Flametongue, gives high Fire and Slash attack. The free VC King of the Lions, Mont Leonis also gives additional offensive stats to increase damage.
Nightblade sub job has Double Resist, and it's quite worth it to use if you're going against pierce/missile type enemies. The rest of the skills are mostly situational, and potentially might not even be used at all, except for Maiming Slash if you want to use him in PvP to counter healers like Yuna. His Ninja sub job isn't really worth considering since his main job already has a decent range on offensive skills.
His EX upgrade is pretty solid. The lvl 19 skill upgrade gives hate on Blessing of Leonis in case his self-buff isn't enough. The lvl 25 new skill, Through Caelum, is an amazing quick ability use that simply gives him more tankyness.
Overall, he's a strong tank with good damage that can help clear content, but he's expensive to max out. He can be somewhat easy to build since Flametongue and King Mont VC are free, and several options when it comes to espers. Worth rolling for if your team needs someone to fill that role.