Again, one last time, God Serena's spells can take Neinhart down due to his attack power. MP only plays a minor factor in this. Think of it this way...
- Attack Power + MP = Damage Output
- MP ≠ Damage Output
You are confusing MP with damage output when really attack power plays a much more important role.
Neinhart didn't use an attack at the time so Brandish was obviously not trying to shrink a spell. The only thing left that she could shrink was Neinhart himself which proved futile because of his immense magic power.