I disagree. If it was not constant damage, just get a rapeload of Ursas to pound the Lucy. That would take care of it.
QFT. Each Harbinger does 3,000+ damage, almost as much as a Sathanas BFRED. If the GTVA built the Collossus to take out the Lucifer, it was likely nothing more than a desperate stopgap. Bombs are the
real capship killers, and if they couldn't kill a shielded Lucifer with bombs then they couldn't kill it with beams. Most likely the Collossus was meant to destroy the Lucifer and its escort fleet in subspace. Shouldn't be too difficult, the GTVA discovered the technology to track "large ships" through subspace from the Ancients at the end of FS1.
Even without invincibility, though, the Lucifer would probably win. Its front beams aren't like anything in FS2. They're not even beams, but rather (in game engine terms) "ultra-long-range dumbfire missiles with a long trail and invisible warhead", to quote the FSwiki. They have a range of 15,000+ and a damage of about 20,000 per hit. Remember how the Galatea was killed by one or two hits from the Lucy? Orions have HP of over 100,000. That gives you an idea of how powerful the Lucifer's beams are. Much more powerful than anything the Collossus has. To kill a Lucifer, you need to either use bombers or approach it from the flank. A head-on charge with a capship, even the Collossus, wouldn't be a good idea.