Sentinel

Sentinel is an optional boss monster located and fought at the end of the optional dungeon Islet Cave in Golden Sun: The Lost Age. Sentinel awaits players willing for a battling challenge and guards the summon tablet containing the Catastrophe summon sequence.

Background and story
Sentinel, resembling a large, animated suit of green armor, does not have any bearing on the game's actual story, nor is any background information provided. It is a powerful entity standing guard within the deepest depths of Islet Cave, the secret dungeon located in the vicinity of the Sea of Time Islet and accessible only by riding a turtle to its above-water entrance.

In the event any group of intrepid and capable Adepts discovers the secret dungeon and has the Psynergy powers necessary to traverse a long series of hallways, Sentinel and the summon tablet await in a dark torch-lit chamber at the end. Sentinel will speak to his challengers via telepathy to introduce himself before attacking and using his powers to destroy the opposition:


 * “I am the guardian of wind. If you would claim my power, you must prove your worth.”

In The Lost Age, as Isaac and Felix's combined party of capable Adepts nears the end of its quest, they may have discovered Islet Cave previously by helping out a lonely turtle, and if they have mastered the Teleport Psynergy then they may end up exploring the cave further and eventually crossing paths and swords with the Sentinel. It will be a fierce battle to wage, but if they are victorious, they will gain the ability to summon the most fearsome embodiment of the power of wind and lightning, the colossal winged entity known as Catastrophe, in future battles.

As a boss
Sentinel is one of four "superbosses" located throughout Weyard in The Lost Age, the others being Valukar, Star Magician, and Dullahan. All of these bosses fight using unique battling setups and are very tough, but Sentinel can be viewed along with Valukar as one of the easier ones. Sentinel is encountered at the end of the optional Islet Cave dungeon, which can only be entered by giving the Lil' Turtle to the turtle at the Sea of Time Islet and riding on him to take you to the cave entrance, and the later portion of the small dungeon where Sentinel is located can only be reached with the Teleport Psynergy found in Mars Lighthouse. Specifically, the battle will automatically begin as soon as you visibly approach Sentinel and he steps towards you to attack.

Sentinel, statistically, has the third highest HP rating in the game at 8736 HP which regenerates 200 HP per turn, and he has the highest PP rating in the Golden Sun series at 780 PP that regenerates 30 PP per turn. He has the second highest Attack rating at 608, 216 Defense, a comparatively low 171 Agility, and 54 Luck. Sentinel is one of few opponents to act three times each turn. Sentinel's resistances are are very lopsided; he is very resistant against the Mercury element and has decreasing resistances toward Jupiter, then Mars, and then Venus-based offenses. Sentinel also carries a unique trait that is shared by no other enemy or ally in the series: Any Psynergy used against him, whether offensive like Odyssey or debuffing like Dull, is instantly nullified and negated, automatically. If Sentinel is defeated in battle, you are awarded 10538 EXP, 6144 coins, and can gain the summon tablet Sentinel was guarding, the Catastrophe summon.

Arsenal
Sentinel's collection of abilities consists of top-of-the-line offensive Psynergy and a strong monster skill attack. Since he acts three times, he will do any of the commands below three times per turn:
 * Attack: Strike a single party member with a single physical attack. Sentinel has the second highest physical Attack rating, but even that doesn't make this command as strong as his others.
 * Searing Beam: One of the strongest area-of-effect Mars Psynergies. It will do moderate damage to most if not all party members. It costs Sentinel 36 PP.
 * Blue Bolt: A rather weak area-of-effect Jupiter Psynergy. It will do only adequate damage to some party members, and will cost him 14 PP.
 * Destruct Ray: A strong area-of-effect Jupiter Psynergy. It will do moderate damage to some party members, and will cost him 21 PP.
 * Spark Plasma: Practically the strongest area-of-effect Jupiter Psynergy. It will do moderate damage to most if not all party members, and will cost him 37 PP.
 * Guard: Increases Sentinel's Defense rating to 125%, and if he uses it again his defense rating is raised to 150%, the maximum. It will always increase his Defense rating by 54, making physical attacks against him less damaging than they should. It costs 3 PP.
 * Break: Sentinel removes all buffing effects that the party members had applied to themselves with Psynergy and other effects at this point. This effectively prevents players from increasing their party's battling abilities to battle Sentinel equally as he increases his own stats. It costs Sentinel 5 PP.
 * Armor Crush: Sentinel's trademark monster skill: He physically attacks an Adept with a spectacular 3D explosion filling the screen on impact, doing strong damage and likely lowering the Adept's Defense rating by 25%.

Strategies
Sentinel, though one of the four "superbosses" of The Lost Age, is considered only somewhat harder as a boss as Valukar, and definitely nowhere near as dangerous and tricky as the Star Magician and Dullahan. This is because Summon spells in Golden Sun games deal damage proportional to the size of each struck opponent's maximum HP meter, so moderate-to-high-level summon spells will always do massive damage on him. And since Sentinel is immune to Psynergy and not summons, this can actually be a very straightforward battle easily won with constant rush-summoning.

Pretty much the only thing you need to worry about is party health, because Sentinel uses Armor Crush and Spark Plasma and similar skills three times a turn, but if your party's levels are in their mid-30s the battle is definitely winnable. Use Jenna's Cool Aura every so often and your party will most likely not seem to go down easily, and if your levels are high enough that Pure Wish can be used, it makes the fight all the easier. The battle is even feasibly winnable through direct physical attacks with weapons like Sol Blade and Excalibur, without the need for any summoning.