One Piece Pirate Warriors 3 - Legends

Prologue Legends
Orange Town - Clear the stage with no allies retreating. Self-explanatory

Chapter 1 Legends
Syrup Village - Clear the stage with no allies retreating. Again, self-explanatory

Baratie - Defeat Mihawk. Treasure mission already makes this mandatory

Arlong Park - Clear the stage with no allies retreating. Again, self-explanatory

Loguetown - Defeat Tashigi in a Kizuna Rush finisher. Unique because she's the only non-final boss enemy in the game that requires this

Chapter 2 Legends
Drum Island - Clear the stage with no allies retreating. Again, self-explanatory

Alabasta - Defeat Crocodile twice. Missable if you spend too much time in the lower territories trying to get an Z ranking. He first appears in the central territory beyond Mr 1 and Mr 2.

Mock Town - Gain 1500 KOs or 750 (!). Again, effortless when going for an S ranking

Skypiea - Quickly defeat Wiper. Easy to get without trying.

Chapter 3 Legends
Water 7 - Okay this is where the descriptions start to get vague. ShonenGamez has it as "finish the stage before construction worker reinforcements arrive" which is both wrong and vague. What you really need to do is time the battle on Iceburg's mansion in a way that Nami sneaks through the back door just as the final wave of enemies at the front entrance are being deployed. Mid-battle saves are a must here because there are several ways this can go wrong. There are a total of 10 waves at the front entrance and you must defeat the first 9 and have the 10th deployed as Nami opens the door. It's possible that you can do this with an earlier wave by clearing it out, then facing Blueno. But frankly it was so annoying and time consuming that I haven't bothered to test other variants.

Enies Lobby - All Strawhat strategies succeed. Another vague one, but easier to figure out. When you cross the bridge and get Franky, go to Jyabura's territory and clear it & defeat him. Then head further to the corridor leading to Lucci and destroy the switch. This will block Spandam temporarily. From then on, capture territories in a counter-clockwise fashion until they are all captured, saving Kaku's for last. Only then proceed to Lucci. This way, Sanji get's the Jyabura area, Sogeking gets the bridge, Franky get's the pantry and Zoro gets the garden. Strategy successful. Then just beat Lucci, save Robin and escape ASAP.

Water 7 Departure - 1. Do not let the pirate with Franky's pants retreat. Easily done going for S or even just your first time through. 2. Defeat Coby twice. Not difficult. Just remember to get him the 2nd time in the SW corner of the map and not just go straight for Garp/Aokiji at the end.

Thriller Bark - Yet again, don't allow any retreats in the stage.

Chapter 4 Legends
Sabaody Archipelago - Kizuna Rush KO Kizaru when he first appears then do it again when he's the boss. Missable, but easy to get without trying.

Impel Down - Defeat Minotaurus 4 times. Easy to miss if you don't keep an eye on him, as the stage tends to reach the boss event by the time it's necessary to put him down the 3rd time.

Marineford Part 1 - Defeat all bosses. Important to note you cannot complete this with Luffy because Hancock refuses to fight you. Also, it's important to go for Hancock & Kuma before defeating Smoker because there'sa good chance the next phase of the stage will start and they will retreat. Also, be sure not to accidentally activate the final boss event at the end.

Marineford Part 2 - Defeat all bosses. Important to note you cannot do this with Luffy or Whitebeard because you cannot take part in the final phases of the stage, namely fighting dual-fruit Blackbeard. Early on save Garp and Kizaru for last. Also be sure to defeat Sentamarou when possible because he tends to stay at the sidelines minding his business. Challenging because most of the characters at your disposal will likely be under-levelled (though can be fixed with beli) and likely unfamiliar to play as.

Final Chapter Legends
Return to Sabaody - Do not allow Perona or Hancock to retreat. Not difficult at all.

Fishman Island - Escape from Ryugu Palace. Sounds easy, but the trick is to know which characters allow this. You can only do this with Zoro, Usopp or Brook. They will begin locked in a room until Brook uses his soul powers to scare an enemy into opening the gate.

Punk Hazard - The last one that requires no retreating allies. This includes not allowing any marine captains to be abandoned in the Shinokuni contaminated areas.

Dressrosa - All strawhat strategies are successful. This one is rather vague but not too hard to figure out. The most difficult portion is early on. Your goal is to get Ceasar Clown to the drop point, but also prevent Bellamy's group from taking advantage while your're gone. So take care of the mission-based captains in the town, all the while spending a bit of time supporting Law. All 4 captains in town must be defeated before Bellamy appears, and he must be defeated before he retreats. If you succeed, you will get a green "objective success" message jsut before Doflamingo appears. If not, you will get a red "objective failed" message instead.