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.

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. In terms of resistance, Sentinel has a Mercury resistance rating of 193, a Jupiter resistance rating of 152, a Mars resistance rating of 127, and a Venus resistance rating of 72, and in regards to its abilities it uses its available Mars-based attacks with a Mars power rating of 120 and its available Jupiter-based attacks with a Jupiter power rating of 110. 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 in the Jupiter and Mars elements, and a strong monster skill attack in the Venus element. He acts three times per turn, with any one of the below battle commands being selected per action:


 * Armor Crush: Used 53 out of 256 times, this is Sentinel's single monster skill, where he physically attacks an Adept and a spectacular orange explosion with 3D fire effects fills the screen upon impact, dealing a Venus-based attack equal to his normal physical attack multiplied by 1.4. There is also a chance that the Adept's Defense rating will be lowered by 25%. This is Sentinel's main offense against single targets, and is quite strong.
 * Break: Used 47 out of 256 times, this is a Psynergy spell that removes any statistical buffs from the current battling party. This ability consumes 5 of its user's PP.
 * Guard: Used 41 out of 256 times, this is a Psynergy spell that buffs up the user, increasing the user's current Defense by 25%. It consumes 3 of its user's PP. Because you can't use Break or Impair to drop Sentinel's Defense due to his immunity to Psynergy, and because he can remove any buffs you give yourself from Psynergy like Impact using Break, your damage output from any physically-based attack will always be lowered for several turns.
 * Searing Beam: Used 35 out of 256 times, this is a Psynergy spell where the user shoots out a large laser-like beam of fire, dealing a Mars-based attack with a power rating of 170 and a range of 7. It consumes 36 of its user's PP.
 * Spark Plasma: Used 29 out of 256 times, this is a Psynergy spell that strikes the party repeatedly with a barrage of purplish-white lightning bolts, dealing a Jupiter-based attack with a power rating of 180 and a range of 7. It consumes 37 of the user’s PP.
 * Blue Bolt: Used 23 out of 256 times, this is a Psynergy spell that strikes multiple Adepts with a quick array of purple lightning bolts, dealing a Jupiter-based attack with a power rating of 90 and a range of 3. This ability consumes 14 of its user's PP. Decidedly weak amongst Sentinel's Psynergy.
 * Destruct Ray: Used 17 out of 256 times, this is a Psynergy spell that strikes multiple Adepts with a thick, thorough array of purple lightning, dealing a Jupiter-based attack with a power rating of 150 and a range of 3. It consumes 21 of the user's PP.
 * Attack: Used 11 out of 256 times, this is this boss' standard physical attack. Sentinel has the second highest physical Attack rating of all enemies, but that of course only means that his Armor Crush skill is all the more powerful and dangerous than this straight attack.

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, Tisiphone Edge (which can be found in the dungeon he resides in), and Excalibur, without the need for any summoning.

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.