Addressing Kingo's points, let's break it down piece by piece:
"Lobo should be at the very least quicker than Zarathos thank to keeping up with Lanterns"
Let's see, Zarathos GR is considered about 48.9 Quadrillion c due to keeping up with Thor and beating him and several other Avengers members with relative ease. Lobo is considered at minimum 87.6 Quadrillion c since he's comparable to Superman and he's faster than Green Lanterns. So yes, Lobo is faster. Is the gap negligible when it comes to punching and blocking? Maybe, I'm not sure.
"...if he has comparable AP then hellfire and penance stare are unlikely to work."
Hellfire is considered not to work since Lobo's been to Hell and loved it. However, the Penance Stare works a bit differently. It doesn't seem to really be based on physical power, but delivers pain based on the sins committed by the victim. Yes, there has been cases where it hasn't worked, the one most relevant being Thanos since he loves pain and suffering. However, Lobo doesn't seem to enjoy being tortured so much that he would be able to shake it off. Considering it killed a physically invulnerable man (
here ) and once killed an angel for telling one white lie, it should be able to kill Lobo.
"... Lobo could easily run off and get a holy weapon or use the spirit-holding gun to contain Zarathos"
According to Lobo's page, a spirit-holding gun isn't in his usual arsenal and Lobo would have to figure out that holy weaponry is his weakness. He probably can, but as they showed that GR outran Mjolnir on his bike and the fastest we've seen Lobo's bike go is enough to outrun a black hole, GR should be able to keep up if Lobo tries to bike away.
"Eating his soul could work, but again, if they're comparable in AP then Lobo could probably just punch Zarathos if he tries to vore him."
I don't think Zarathos has to physically touch him to consume Lobo's soul. In the comic panel they showed in the episode, yes, Zarathos is holding the guy still, but the soul is being sucked out from his body, Zarathos isn't actually eating his flesh to get it.
"The problem with Zarathos is that most of his power comes from hype from people like Strange. Whenever he's actually fought a universal being, such as Mephisto, he's been amped."
True, the first time Zarathos fought Mephisto, he was amped by his followers. However, a Ghost Rider host has beaten Mephisto by themselves before and Zarathos has fought Galactus, who is a universal character when decently fed, and their fight shook the universe as an afterthought. If either of those instances involved amping, let me know.
So there, that's my thoughts on the matter currently.