Sentinel

Sentinel is an optional superboss in Golden Sun: The Lost Age, located in the Islet Cave. It is known as the "guardian of wind".

Battle stats
Sentinel has 8736 HP which regenerates 200 per turn, 780 PP which regenerates 30 per turn 608 Attack, 216 Defense, 171 Agility, 54 Luck and can attack 3 times per turn. Though it's element resistance is fairly balanced, it is weak against Venus attacks and most resistant to Mercury. When slayed, Sentinel yields 10538 EXP, 6144 Coins, and access to the Catastrophe Summon Tablet.

Sentinel has a number of powerful psynergies including: Searing Beam, Blue Bolt, Destruct Ray, Spark Plasma, Guard and Break. He also knows the monster skill Armor Crush. He is unique in that he can nullify any and all psynergies directed at him, and his high defense, plus his tendency to cast Guard very often (which can't be dispelled by Break, thanks to the aforementioned psynergy immunity), makes it hard for physical attacks to scratch him as well.

Battle strategy
Most players rely heavily on summons to defeat Sentinel. Bring a Mars Adept (preferably Jenna as a Justice, Radiant, or Paladin, as she can do auxiliary healing when things turn ugly), both of your Venus Adepts, and a healer with Wish. This way, you can summon Haures every two turns, which can deal 2000+ damage easily. Try to use Ground and Petra heavily, and if Sentinel focuses on using Armor Crush and Blue Bolt rather than his more dangerous arsenal of attacks, you should be able to win relatively easily.