Thunder God is a top-tier physical slasher. He has a variety powerful ranged attacks, among the most useful is the Armor Crusher, which sinks its teeth against high defence tanks. He is also beefy enough to take several hits, even tanky depending on build. Use him as a main/alt attacker while a tank aggros hate.
The Samurai sub-class is recommended, as it offers a wider variety of Buff spells, which allows to greater AP gain before engaging in combat. The best Esper for him is Odin.
Unfortunately, he is a limited-time event unit that has already transpired. There is a possibility of obtaining more shards for him in the 2nd Final Fantasy Tactics event if you possess him and have not finished limit breaking him.
He's weak to earth elemental, magic and piercing. Currently released units like Aileen, Murmur hypothetically could counter him. If Mediena catches him with a flare that could also end him.
a defensively built Orlandeau can eliminate his magic weakness
Smart coat, Fenrir esper/card, low faith
Since Kilphe uses things like Flare, Holy, etc (that don't do earth damage) its not unusual to have Cid survive 3 or more hits from his "counter" - more if he has a barrier and heals
while Kilphe might use more powerful ranged attacks at a distance like Holy or Flare that do not have earth elemental, she would still have 25% type advantage over him which is calculated in all instances of damage.
to elaborate on this just a little further, because she is of earth type and Cid is of lightning type, any time the two units engage, Cid would deal 25% less damage regardless of his attack element and would receive 25% more damage regardless of the attack he would receive from Kilphe.