Remilia

Remilia is a powerful late-game unit with high attack and speed, even more so than Sakuya's. Her unit skill converts 1/3rd of the damage she deals to opponents as health, making her impressively self-sufficient. Many of her spell cards also have regenerative effects besides her unit skill. Her one real weakness is slightly low magic defense, but it should be noted that her level growth is very slow.

Where to Obtain
Remilia and her spell cards are acquired from the Main Quest "Scarlet Devil Mansion Rooftop," which can be cleared by units level 30 and above on Easy difficulty

Strategy
Remilia may be one of the most self-sufficient units in the entire game-- she doesn't have many spell cards that assist the entire party like many other units do, but she is very effective at keeping herself alive. Star of David and Red Magic are very good at drawing health for Remilia from multiple opponents, but they are less effective when there is only one enemy, or there is a Meiling present that is drawing all the attacks to herself. Star of David is also a strong lead-in for Red Magic, and using the former first for a time will increase the effectiveness of the latter once the player has accrued enough bombs to use it. Scarlet Shoot's effect will stay active even if Remilia casts another spell card over it, so using it in tandem with Star of David will allow Remilia to regenerate large amounts of her health with each attack. Both Scarlet Shoot and Red Magic benefit greatly from Reimu's Fantasy Seal, which can send a large amount of activated spell cards to the Used pile all at once. Young Demon Lord's power is double when its special effect is in play, so if Remilia is easily regenerating her health over her opponents', the card can quickly decimate even slightly weakened targets. But Scarlet Gensokyo is still Remilia's strongest card, with both its effect that can render an enemy team helpless after a Fantasy Seal clear and its large amount of multi-hits from the get-go that make it well worth its high bomb cost. It is slightly ironic, but Remilia shouldn't be a target for Sakuya's help most of the time, and she doesn't have a lot of obvious partners besides Reimu and perhaps Patchouli. However, when considering that her role is mostly to be a damage powerhouse, it makes it more understandable that other units would be providing support to her and not the other way around.