I belive Segunn is referring to the boss at the end of Chapter 1 at the Kilika Temple.

Level 10 hmm, that's kind of low. Here's my advice (besides go level up). Have 3 Warriors, all with Sentinel. Have two of them cast Sentinel repeatedly and a third start attacking. This way, Haymaker will only do 2 HP of damage to the person who is in Sentinel, and hopefully the attacker won't get hit. If the attacker gets hit, switch her to sentinel, have another start attacking, and heal when you can. If you have the ability, use power break once to weaken it. And FYI, for some reason Thunder magic doesn't hurt machina and mechs as much as it did in FFX, I don't know why.