While Guardians are strong and can be revived, they wouldn't last long against Space Marines. A Space Marine Librarian is described as being able to generate force fields and blasts of energy, so one of them covers Warlock and Titan abilities. The whole Guardian revival thing is dependent on Ghosts. We come across dead Guardians in the game that we can't revive, it's implied our individual ghost has to be working to revive us. Once this was noticed, they would be the primary target. We know the ghosts aren't made for combat, it asks us to cover and fight for it every couple level. When comparing vehicles, it's very lopsided. Guardians only have Sparrows and Jump Ships (the other two useable vehicles are Fallen and Cabal). While the ships look armed, we don't know if they are and what they could be armed with. The Space Marines have various tanks, bikes and massive space ships. Now both sides have a motionless near omnipotent figure they rely on for protection. Both were critically injured in a massive battle. Compare them in their current states after being injured. The Traveler floats over Earth, protecting the last city of Humanity while somehow the Speaker interprets his will. The Emperor sleeps on his life sustaining throne while his mind fights the Chaos Gods holding them off while directing the massive Imperium. While I enjoy playing a Guardian here and play an Ork army in 40k. I have trouble seeing how a fight between Guardians and Space Marines can end well for the Guardians.