Cool matchup.
I think based on the lyrics of Judas Priests' song Painkiller (“faster than a laser beam”, meaning that he is faster than light, and “brighter than a thousand stars,” meaning that his magnitude of brightness could easily make someone go blind or even vaporize, “Planet devastator,” meaning that he could destroy planets, etc.), he scales to at least Planetary, possibly Large-Star level (High 4-C, I think), but Spawn at his peak is possibly around 2-A with max hax.
It depends on which version of Spawn you're talking about. Painkiller could beat him easily without his Green Powers, otherwise, with his max hax, Spawn wins.
