Artyom would win this because he can sneak fairly well and he is immune to anomalys (( if any appear in this battle Yar is fucked )) if he can sneak up close enough to use an AK on him or a Shotgun he wins instantly but if it comes down to a sniper battle Artyom still has the advantage with silent pneumatic weapons and nightvision googles he still has the upper hand
SO ya
WINNER:
Artyom
Ahem.
Uncle Yar has sneaken away from multiple things due to his days as a sniper, and is quite the pistoleer also. Pneumatic weapons such as the Tihar would do nothing against a Guardian of Freedom suit, and Yar would simply ambush Artyom. In addition, Yar has dealt with similar anomalies in the Zone, and the only thing you need to do to survive a Metro 2033 Anomaly is stand still. His suit is also double-layered, specifically with Treated Leather and Asbestos yarn, rendering him able to survive plenty of anomalies and radiation. Should it be fully modified with Anomaly protection, he is immune to almost all environmental hazards. As for bullets, the zirconium reinforcement would simply take up the Shotgun, and Yar
is an expert STALKER, so he is probably skilled with assault rifles to take one apart, put it back together again, and make it able to use 9mm rounds.
Artyom is a CQC specialist, due to his skill with the knife, and the fact he is usually stuck in the dark corridors of the Metros. His Bastard SMG would probably end up broken due to it's poor craftsman ship, as would is Duplet. His Kalash, VSS, and Kalash 2012 are the only possible effective weapons on Yar, as the Hellsings accuracy degrades at range.
As for team up....
Yar is not wearing the uniform of Communists, Nazis, or Bandits in Artyoms world, and to Yar, Artyom would look like a Loner with his canonical Heavy Armor and his starting armor. So if they meet, there is a good chance they won't shoot each other. Yar would be the one to ask the questions, and Artyom would listen or respond quietly. Eventually he'd end up following Yar for protection as Yar had enough experience with correctly scavenging as a Stalker and has much higher accuracy with weapons.