Strategy to beat Lingering Will?

Dredging up the gamefaqs comment that helped me a few months ago...

The Fenrir jump method

Summon Stitch, and ride Once More into free ukulele openings (the one he does when Sora is in Critical HP, not the Limit). It also makes him drop HP balls in addition to stunning him, so getting near him for the opening (use air combos) will get you your Second Chance and Once More back. You'll need to Glide the first 2 segments of his DM, and not much else. If something goes wrong, and you have 1 HP, but Terra isn't stunned (mainly due to the Glider), just Cure, Stitch will refill your MP in a sec. Doing damage increases Summon time, so you shouldn't need to resummon much, if at all. Getting hit by an Attack Seal can really mess this up, since Stitch auto deflects Ultima Cannon orbs (but it's harder to get those back at Terra), and the Ukulele can keep you from getting to the Finisher in a Thunder string. Probably best to take off Ground Combo Pluses, but I'm never tried that with this method. Can't think of any reason not to, though.

Open with Donald's Comet, in the first opening, get him in the air (can't remember if Upper Slash works, if not Finishing Leap), when Berserker Charge Horizontal Slash. If you mash the crap out of Square it can beat his counterattack (also needs some luck), and you can keep it going until he dies. This one works best with softcapped stats, so that you kill him before you get your MP back. Most people use a Turbo controller for this, but it is possible without.

None of those work at level 1. The easiest way there (and still works great for high levels, just not broken-good), is the loop Thundaka mentioned. If you do it right, LW will mostly only use Sliding Dashes, Keywhip and his DM, and you'll know they are all coming.

That loop he is referencing is here:

That fight is all about manipulating his counterattacks. For the first 2/3 of the fight, you want him to counterattack basically all the time: use three long combos with one Explosion each, Dodge Roll out of the way of his flying slashes and whip attack (he typically counters with three slashes+keywhip, though at times he'll perform 6 slashes in a row instead), and move in as he lands to start it over again. The last 1/3 of the battle is trickier, but still becomes more manageable if you stop just shy of your third Explosion: that will trigger the desperation attack every time, and if you have a good plan of attack for surviving that move the rest of the battle will be easy.

When I first faced him, I swore the fight was impossible myself. It was only when I came back at Level 1 that I realized how it easy it was to loop him.

This was enough for me to get the courage to try these a hundred times until I nailed it. Perseverance, my friend. Good luck.