I used this strat for Yuuka on hard, practically fail proof on hard and below. Have these ready:
Anyone that is a hitter in this stage must have Fury, otherwise they cannot hit Yuuka for shit.
Marisa with PS: Danmaku Power maximum, throw a Makai Grimoire on her.
Eirin with Resupply (the Passive) and Resupply (the seishin)
Elly or Yaname with Tile Damaku, or Spider Web respectively.
Aya/Yukari with the teleport ship combo. Aya with Tengu Feather, and Tengu clogs, Yukari with Spiriting Away PS (Warps any unit towards Yukari)
Use Fairies/Prismrivers separate (not the leaders) as free units to trigger Support Attacks, or use bombs.
Turn 1, and 2 hundle around together, use Meryrin's power up/Yuyuko + Doll crashing Dance combo to gain quick power.
Aya/Yukari combo fly away with Yukari casting Hide every turn, collect the bomb from the fairy on the way up.
Pile everyone on the ship by the end of Turn 2. Get Aya/Yukari very near Yuka.
Turn 3, warp the ship over, deploy everyone and hundle around Yuuka, put everyone on the line, or above Yuuka.
Put her at low health, but not start her spell. On higher difficulties, be sure to use tanks as the front as she will use Master Spark with Valor and Strike.
Turn 4, start the spell, get the WP. Spell effect is: Yuka can pass unit placements, and all your defensive casts are useless. Use a bomb, and attack, but again, do not kill her. If you did not hit Yuuka with the bomb, be sure to use Elly or Yaname to stick her in place.
Turn 5, Break Gensokyo's Reflowering, Double Spark makes 3 more Yuuka clones, Align Marisa and other sniper unit on one side, Reimu's team and Yorihime's team on the other side, wait there. Rest of the forces (except for 1 unit) MUST take out Top Yuuka, otherwise you will eat a Double Spark MAP in the face. The last unit NEEDS to bomb the other middle Yuka, otherwise she makes more clones. Pass the turn.
Turn 6, Use Marisa's Damaku Power Maximum, cast Courage, you should be able to OHKO the Yuuka clone, if not, the 2nd sniper you have for safety can get her. alternatively, use Full Power mode. Have Reimu team and Yorihime team take down the last Yuuka clone. Finally, kill the middle main caster Yuka. Bomb Yuka off Nightmare Spark's effect for safety, but you can safely take her out that turn.
If you did everything correctly, you would have no MAPs that hit you pass here if you never put any units south of Yuuka. Be sure to stick her in place during Beauty of Nature, as she likes to move around that spell, and just bum rush Four Seasons Flower Master.