A Direct Action merc sleeved into a Reaper morph should do the trick, even with the kind of anemic gear he has. (Using the premade character from Sunward as a guide)
Round 1: Without any equipment he's a very strong and fast human. Without any equipment the Reaper is still the heavily armored bastard child of an attack helicopter and superhuman blender with daggers for fingers (when it wants them). He can't outrun it, as it can fly, and he can't hurt it before it uses one of its four tentacle limbs to take his head off. He's fast, but the Reaper's "nervous system" is made from room-temperature superconductors giving it a truly beyond human reaction time. (no hard numbers though) Between its augmentations and the natural abilities of its Ego, it has the fastest possible reflexes available to Transhumanity, and it probably FTE with speed to spare.The portals and teleport only give him a few more seconds before the reaper mauls him. Human martial arts are of little value vs an armored 4 limbed smart material disk, so close combat is out of the question.
Round 2: He gets some guns which don't pose a threat to the Reaper, some grenades which might hurt it if it doesn't catch them and throw them back, and a sneaky suit. The reaper gets a very futuristic rifle with some fancy ammo, and what amounts of a 40k boltgun of steroids (seeker rifle). The sneaky suit is of little value, as the reaper can see the entire EM spectrum (radio through gamma) in all directions at all times. He dies quickly as a brace of homing hypersonic HE warheads liquify his body.
Round 3: A week of prep won't give him enough of an advantage to bring down the reaper, and EMP won't effect it. His best bet is to get some really high power weapons, but that doesn't help him with the problem that the Reaper cannot be hidden from. It's own week of prep will give it some more advantages due to its great ability to collect intel or set up an ambush, it also doesn't need to rest or sleep, so sneaking up on it is yet harder.