Initially, my gut told me Anton. But my brain says Mike. Mike has had to contend with dozens of other assassins, gangsters, and people of ill repute. And that's just since he left law enforcement. Protecting a principal from cartel hitters has given Mike daily experience in staying ahead of savagery. Law enforcement experience makes him highly knowledgeable in modern TTPs.
Anton, on the other hand, makes a living out of taking out non-professionals. He's ruthless, relentless, and smart to be sure, but I don't think he has it in him to hunt down a man like Mike. Remember: Mike has SOCOM combat vets to back him up, as well as law enforcement resources. Anton would be trying to hunt a pride, as opposed to a single lion.
Me thinks that Mike doesn't even bother going on the offensive. He waits for Anton to come to him, then makes him play on Mike's court. When the actual shooting starts, Mike and his cadre cut Anton down swiftly.