Galahad's main artifice is her heal on CX, which is... rather problematic, to say the least. Her secondary artifice is her heal reduction, which invalidates the possibility of using heals such as Awakened Shiori. Not that healing would be much use if you can't kill her.
Galahad is immune to MaxHP% DoT.
Heal Reduction, provided by Stage Girl Futaba, or Brilliance Reduction are both fine mechanics for dealing with her. Alternatively, you could nuke her off the face of the earth with an overwhelming amount of damage, using Cheer Yachiyo's exit evasion to cast multiple CXes, or very highly buffed Tsu the Crane, Merlin, or Awakened Tamao.
1) Healing Reduction
Before it's asked - no, Red Duke Claudine's 41% HP Recovery Reduction is not going to work. Stage Girl Futaba is necessary. Bring a conventional space team with 1-2 tanks, 1-2 attackers in addition to Futaba, and then whack her.
Sample 1- Seishit
2) Brilliance Reduction
Since Galahad's healing is mostly tied to her climax, using brilliance reduction to prevent her from climaxing is a viable strategy.
3) Brilliance Reduction plus Necromancy
Incessantly preventing Galahad from climaxing, while triggering exit evasion over and over again. Do not at any point in time use Yachiyo's evasion chance up move.
1) 300,000 HP? That's nothing.
Refer to Nuke Theory - Space has a large variety of options with which to oneshot Galahad.
2) Exit Evasion Shenanigans
Lack the unit skills or girls to delete Galahad in one strike? No worry, just make sure you have a Cheerleader Yachiyo + 1 space chopper of your choice, and three unit skill sticks. If the space chopper is in front of Yachiyo, you will only have one attempt; if the space chopper is behind, it is more stable. This is because Yachiyo can chain-cast her CX successively if she is repeatedly defeated, but if your designated damager is in front, that won't work. You will thus have to activate Climax Revue the turn before you use the chop. On the next turn, have the damager use her climax act (after Galahad uses hers), activate exit evasion, and then the turn after use the climax act again.