...I beat it through sheer luck just now. Mainly, it's timing the use of grief seeds on people who are about to die, and just landing really hard hitting attacks.
That said, Sayaka died because her soul gem ran out.
To people who can't beat this, either refer to the comments of the previous post of this, if it doesn't exist anymore (I haven't checked and won't bother ... (yes, I'm a lazy prick, so what's your excuse (lmao))) I'll provide some advice (or just make an entire translation (I'm bored, and I need practice in my Japanese). First off. First options in the beginning of battle. たたかい (Attack) へんせい (Organization) Straight forward options, then: In organization: パーティへんさい (Party Organization) まどか Madoka ほむら Homura まみ Mami さやか Sayaka きょうこ Kyouko けってい (Decision, meaning you've made your choice in character selection, but it's useless when you pick the third person, only used if you feel the need of challenge and want to use 1 or 2 characters at a time) もどる (Back, retracts the last selection so you can choose again) キャンセル (Cancel, do I even need to explain?)
After making your party selection, click たたかい and then you have: たたかい (Attack, uses standard attack) まほう (Magic, you don't need to be a magician to know what this is ... *further insight soon*) にげる/ジャンプ (Escape/Jump*, useless command as clicking this just makes you look like the retard pressing the buzzer on a quiz show. *Jump is exclusive to only Kyouko, and if you've played Final Fantasy and know what a Dragoon does then nuff said. To those who don't (why you even playing this then?), Kyouko temporarily leaves the field for the turn it is selected, and on the following turn dives down on Walpurgis and does damage) アイテム (Items, apparently in this version, the creator added items for use if need be)
Now onto the magic show:
Madoka's Magic: たばねうち (Tabane uchi (not sure about translation), uses 30SG and does around 1900 damage) ほむら (Homura*, requires Homura to be present in the party and uses up her turn, uses 30SG of Madoka's and 10SG of Homura's, does damage around 6000) まどかのねがい (Madoka's Wish, uses half of her current SG (Soul Gem power) to deal damage variable to how much SG she has left)
Homura's Magic: M249 (M249, requires no SG, deals damage) パイプばくだん (Pipe Bomb, uses 10SG, deals damage) こうそくどうさ (High-Speed Operation, uses 25SG, and bombards Walpurgis with her arsenal of weapons for big damage) まどく (Madoka*, requires Madoka to be present in the party and ues up her turn, uses 10SG of Homura's and 30SG of Madoka's, does damage around 6000)
Homura's Items: グリーフシードSx3 (Grief Seed Small) グリーフシードMx2 (Grief Seed Medium) みだれうちセットx2 (Set of something (:/), later says she's using RPG-7, does damage obviously then) はくげきほうx2 (Mortar) てっとうx1 (Pylon, apparently uses 20SG) タンクローリーx1 (Tank, uses 25 or 30SG, wasn't paying attention) SSM-1x1 (uses 20SG) C4x1 (uses 25SG)
Mami's Magic: ちりょう (Treatment, uses 10SG, recovers selected member's HP) リボン (Ribbon, uses 10SG, not sure what it's supposed to do, but if I had to guess, it reduces old-witch-face's speed, but that's an assumption) みだれうち (Midare uchi (not sure about translation), uses 20SG, summons 6 of her flintlock rifles and deals damage) ティロフィナーレ (Tiro Finale, uses 50SG, and does big damage)
Sayaka's Magic: いやしのひかり (Healing Light, uses 50SG and restores 2000HP to entire party including reserves) みだれなげ (Midare nage (not sure about translation), uses 40SG, and summons 6 of her cutlass and throws them for damage) メッタぎり (Mettagiri (not sure about translation), uses half of her remaining SG and goes berserk (not status-wise) doing huge damage)
Kyouko's Magic: けっかい (I would assume Barrier, uses 30SG, and as the name suggests, supposed to dampen the damage you take, however, it's effects are only targeted on your party, if you switch members, I believe you have to apply it again) ちょうはつ (Provacation, uses no SG and I believe has higher priorty so it's used first to draw Walpurgis' attention towards Kyouko, however this is useless when Walpurgis starts spamming an attack that hits entire party) とつげき (Assault, uses 40SG, damages Walpurgis at the same time suffers from recoil)
Now for strategy, first off you should know that when Walpurgis attacks one of the girls, they lose SG equivalent to 5% of the damage they took. So now, send in Madoka, Kyouko and either Homura or Mami. Save Sayaka for later. Use Madoka's まどかのねがい (3rd magic) as it'll deal roughly 8000~9000 damage right off the bat, Kyouko should use her first magic to dampen the damage, and either Homura should use こうそくどうさ (3rd magic) or Mami uses Tiro Finale. When your girls are starting to wear out, switch out Sayaka and use her first spell to heal the entire party (including reserves as I mentioned). Also, if one of your girls happen to reach 0HP but still retain some SG, no worries, Sayaka's Healing Light can still revive them when they're in reserve. Now, here's the important part, use Sayaka ONLY for her healing, as she's the most potent healer in the group and it's also important when Walpurgis flips right side up and starts spamming her attacks that hit your party for 2200~2500 damage (that's without the barrier, that's why I recommend you keep barrier up every other 3rd turn)
That's about it really. Walpurgis should have like 50,000HP. At least that's what I remembered someone calculated from the previous version. With what I did, all my girls survived.