The four spirits do four completely different jobs
The intro covers how you call them out of the scenery. What matters in a fight is that none of them overlap. Dytto is your medic and your crowd control: tap L and a healing halo chases you down, press L to freeze an enemy solid, and with enough gems a single Water Ball locks a boss in place long enough to empty a whole sword into it. Keep her out against anything that hits hard.
Efreet is cheap and aggressive. His Fiery Punch swings on its own and barely touches your SP, so he can stay summoned for an entire room, knocking down charging Knights while you deal with everything else. Shade is the survival pick: his Bodyguard soaks the damage meant for you, one hit at a time, and that is what carries you through the late bosses. He also reaches out with a claw to drag distant chests over and tear out grates you can't otherwise reach. Bow is the odd one out. Rooted wherever you summon him, he earns his keep by chewing through the grate into the Palace Dungeon and stunning a packed room with Poison Pollen.
Every spirit hides 15 colored gems, 60 across the whole game. Each gem raises that spirit's power and adds +5 to your max SP, and they take up no inventory space, so grab every one you walk past.
Spirits drain SP just by being out, so learn the passive refills. The Psychoring quietly tops up your SP whenever no spirit is summoned, which makes it worth dismissing Dytto between fights instead of leaving her hovering. The Sunburst Pendant heals HP when you stand dead still in sunlight, and the Sun's Charm later does the same for both bars. All three work passively in the background, so you rarely have to burn a real consumable just to top off.
Five infinite weapons, and the mini-games that hand them over
Every weapon except the starting knife burns through its uses, which makes the five infinite versions the real prize of the game. Each one is locked behind a side challenge with an exact condition, and missing that condition by a hair gives you nothing.
The Infinite Fire Bow comes from racing Efreet five laps on his hidden jungle track. You need first place, under 1:09.9. The Infinite Atomic Bow is on the Secret Islands, reached through a Warp Door, and the rule there is brutal: clear the whole area without opening a single chest or breaking one box. The Infinite Hyper Bomb waits at the end of the Secret Crags, a five-screen jumping gauntlet over disappearing tiles and wind-blown platforms. The Infinite Metal Bow sits in the Hidden Waterfall, which you can only enter with Shade. And the big one, the Infinite Omega Sword, is your reward for clearing all 100 floors of the Secret Pit hidden behind a lone tree in the Dark Forest: no spirits allowed, no items, roughly 45 minutes of nonstop fighting if you're good.
How do you beat the bosses that actually stop people?
A few fights read as impossible until you spot the one trick each is built around. The Aqua Crab is the first wall: ignore the body and kill the claw first with Multiple Rolling Slashes, then watch its shadow to dodge the jumps. The Old Dragon is all about her face: that is the only spot open, and Efreet does nothing here, since she's fire-attuned. Summon Dytto instead, let her lunge, freeze her with a Water Ball, and slash from the gap between her head and the cliff edge.
The Non-Entity hides behind a shut third eye that no normal attack can touch. Shoot it with the Light Ball to force the eye open, unload your best sword, then run north the instant its vortex starts dragging you in. Agito, the final boss, only exposes his head when no tentacles are out, so kill the regenerating tentacles first or your hits just ping off him the way they would off a Knight's shield. Keep Shade summoned for every one of these fights. One extra hit absorbed is often the whole difference between finishing the pattern and starting the room over.
Why your strongest weapons keep running out
The knife is the only weapon you never lose, and the game quietly punishes you for forgetting that. A sword spends a use every time it lands a hit; bows and bombs spend one every time you fire, hit or miss. So the player who lobs Atomic Bows at common Soldiers turns up to the next boss holding a knife and a lot of regret. Save the limited weapons for bosses and Knights, and clear ordinary rooms with the blade.
The knife runs deeper than it looks, too. Tap K for a quick Stab, press it without tapping for a Slash that knocks enemies back, and learn two moves above the rest: the Running Slash (run, then press K) that floors a target and opens a combo, and the Rolling Slash (Forward, Back, Forward, K), which lands several hits and is at its nastiest when it pins an enemy against a wall. One more habit pays off all game: kill Rats and Ogres with fire, whether from a bomb, the Omega Sword, or Efreet, and they drop Steak and Roast Beef far more often, which is how you keep your health bar full. And don't waste the Spirit Calls just summoning their spirit. Drop one with Enter and pop it with a Light Ball, and it transmutes into something better: a Lamp becomes an Omega Sword, a Vial becomes a full-healing Roast Beef, an Onyx Stone becomes a Heart, and a Seedling becomes an Elixir. You can play the whole quest online in the browser, so it costs nothing to duck into the Dark Forest and drill these moves before a fight that matters.
Hearts make you stronger, but they cost you your final grade
There is no experience system here, so the only way to grow is to farm hearts, and the method is deliberately uncomfortable. Drop your health down to about an eighth of the bar, let an ordinary enemy land one more hit, then kill that same enemy, and it leaves a heart behind. Each heart you pick up raises your max HP by two and permanently bumps your attack strength. Around ten of them roughly halves the difficulty of everything that follows.
The catch: every heart also raises your Rank, and the end screen scores a lower Rank higher. So the same hearts that carry you through the back half of the game quietly drag your final class down toward Fighter and away from the titles worth chasing. If you only want to reach the credits, farm freely. If you care about the grade waiting at the end, take just enough hearts to survive and stop there. It's a real trade-off, and the game never once explains it to you.