UPDATE: Its recently come to my attention that its possible to prevent Lexaeus from ever powering up, making the fight much easier, since he will never use either grab move or even his DM. Essentially, he doesnt have a typical HP threshold like most bosses. Instead, after passing said threshold, any standard physical attack or magic, like Reflect bullets, will trigger the power-up. Fortunately, the RC does not trigger it, so as long as you only attack with the RC, avoid getting hit (youll need Dodge Roll, unfortunately), and only combo while his Aura is down, you should never trigger a power-up. Just be careful that Goofy doesnt accidentally clip him with the shield, or that will also trigger a power-up (summoning Genie can help prevent this). This video shows how I defeated Lexaeus as soon as his Absent Silhouette shows up. This, of course, means no Dodge Roll (even though Limit Form is literally the very next thing you get). As with Larxene, I show my setup at the end of the video. Sora: Lv. 28 Keyblade: Heros Crest (now that we have Finishing Leap, its much easier to setup air combos) Abilities: TURN OFF QUICK RUN! Quick Run Lv. 1 has no i-frames! If you accidentally use it when trying to guard his counterattack, you WILL die! Also, while Aerial Spiral and Finisher are normally bad skills (due to leaving you completely vulnerable), Lexaeuss counterattack is so slow, that youll still have enough time to land and guard before he lands his hit. Equipment: Buster Band is optional, so no need to go out of your way to craft it. Without it, everything one-shots starting phase 2, rather than phase 3. Everything else can be whatever you want. If you want Majestic Mushroom, though, you can get it easiest at this point from Land of Dragonss mushroom using Wisdom Form. Items: If youre feeling more brave, you can use less Hi-Potions and more Ethers. Otherwise, fill Donald and Goofy with Ethers. You should have enough Munny at this point to buy more at Twilight Towns store if needed. Using Genie is optional hes just a safety net for the healing in case you take damage. Oh, and dont forget to turn off Donalds Cure (like I did, whoops), so he doesnt waste Ethers on himself. For how easy of a boss he normally is, its surprising how difficult Lexaeus gets at low levels. First off, without Dodge Roll, youll have to use Reflect a lot more. This is why all those Ethers are necessary. Fortunately, most of his attacks are possible to see coming and react to, keyword being most. He has 2 jerk moves that come out fast. The first is his charge move. He always starts phase 2 with this move, and will occasionally use it more later if you are some distance from him (which is why its best to always stay close to him). The second looks similar to his ground shockwave attack (the one you can block for the RC) at first, but changes into a spin. That move is the reason why I spent phases 2 and 3 pause buffering, since its near impossible to react in time (although he did not use it in this run). Despite both moves being grabs that power him up, the only way to dodge them is to use Reflect (how youre expected to figure that out on your own is beyond me). Even though its possible to react to most moves, you usually want to hit his revenge value with every combo, so that he always counters with the jump attack. Unfortunately, this value increases in phase 3, so you may need to tap him with Thunder magic to trigger the counter. Just make sure you wait until he lands, because he sometimes delays a bit, then counters before he hits the ground (see the 2nd phase 3 combo). Finally, unlike Larxenes bonuses (An item slot for Donald. Yay?), defeating Lexaeus gives Sora an Accessory slot, so its very much worth beating him early.
Kingdom Hearts 1.5+2.5 ReMIX - Absent Silhouette Lexaeus (1st opportunity)