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. The battle will automatically begin as soon as you visibly approach Sentinel and he steps towards you to attack. After defeating Sentinel, the player can access the stone tablet Sentinel guards, which grants the player the ability to summon Catastrophe in future battles.

Sentinel acts three times per turn, and regenerates 200 HP and 30 PP per turn. Uniquely, he is immune to all Psynergy used against him, whether offensive like Odyssey or defensive like Dull. Sentinel's collection of abilities consists of top-of-the-line offensive Psynergy in the Jupiter and Mars elements, and a strong monster skill attack in the Venus element. He can choose from any one of the below battle commands per action:

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. The main catch to this particular battle is that Sentinel is immune to Psynergy. This means that the player cannot use Break to nullify any statistical boosts Sentinel gains; in addition, Elemental Physical Psynergies like Odyssey are useless.

However, Sentinel is still vulnerable to Summons, Djinn, and weapon unleashes, with Summons being a particularly excellent method of combat. This is because Summon spells in Golden Sun games deal damage proportional to the size of each struck opponent's maximum HP meter. Since Sentinel has one of the highest HP meters in The Lost Age, moderate-to-high-level summon spells will always do massive damage on him. In this manner, this can actually be a very straightforward battle easily won with constant summon-rushing. Haures is recommended due to Sentinel's low Venus Resistance, although those who have already visited Anemos Inner Sanctum could also use Charon. If choosing to use weapon Unleashes, physical attacks from the Sol Blade, Tisiphone Edge (which can be found in the dungeon he resides in), and Excalibur are recommended due to their damage-multiplying abilities. Keep in mind, however, that Sentinel can nullify any Attack boosts granted by Impact and the like with his Break Psynergy.

Another way of beating Sentinel is to alternate usage of four of the party's Djinn: Luff, Rime, Corona, and Iron. Luff and Rime may be used to limit Sentinel's damage to the party to only a physical attack and his Armor Crush (similarly to how Luff limited Deadbeard's damage to the party to only physical attacks in the original game). Corona and Iron would serve as a countermeasure against Armor Crush. Then, while Sentinel is busy trying to break through the party's Defense, one can use Unleashes from either damage-multiplying weapons or offensive Djinn. Note that the Binding effect and the Defense increase need to be reused about once every two turns for this strategy to work. The process will require some healing of consistently targeted Adepts with lowered defense (which should in that case be supplemented by the defensive Djinn), but it will definitely not compare to the healing required if Sentinel was allowed to use his devastating Psynergy offenses.

The main worry for this battle is party health, due to Sentinel's wide array of offensive Psynergy and the powerful Armor Crush, which can be used up to three times per turn. Jenna's Cool Aura is a recommended healing option for lower-leveled parties. If level 46 or above, Pure Wish will make the fight even easier due to its higher healing capabilities.

Trivia

 * Sentinel has the third-highest HP rating and the second-highest Attack rating in The Lost Age.