Superman is faster than light. Celestia is dead before she realises anything is wrong. Unless you're reffering to one of the weaker versions of Superman, in which case you need to specify.
When I say Celestia was "holding back", I don't mean she could have used a more powerful attack (I assume that it
was her most powerful beam), I mean that she just brute forced rather than using strategy, like raising the temperature of the room several thousand degrees, summoning a horde of tentacle beasts, creating a rain of swords etc etc etc.
Celestia's strength is in the octillian ton range, but the more powerful version' of Superman have it at least three orders of magnitude higher. Celestia's only demonstrated quadruple digit mach speed, rather than the billions of light years per second that the more powerful versions of Superman can achieve. Celestia's destructive capacity is a little over a hundred times what's needed to blow up the solar system, but her durability isn't much higher. Superman has dealt with
far more powerful beings.
As for the silliness of Superman's powerlevels, it's not the powerlevels that matter. It's what you
do with them. (Admittedly, I never liked how stupidly powerful Superman was compared to most of his foes.)
Medaka Box proves that ridiculous power levels can be fun (of course, DC has higher power levels than
Medaka Box does).