Heike is a Power Level 280 Zealot, and he can be found wandering the roads of Eurviscire.
Callin wields a sword which he can make poisonous, so that when it hits you, you’ll take a lot of poison damage over time. He also throws poisonous grenades and holds health rations to heal himself up. If you have trouble with the amount of damage you take via poison, you could try putting some points into specific poison resistance skills.
Callin uses a blend of strong (red) attacks and lighter (orange) attacks. Lighter attacks can be parried, but strong attacks should be dodged at all costs. If you equip the skill Brush with Death you can trigger slowed time periods by successfully dodging an attack, allowing you to get in some uncontested hits.
Hitting Callin’s weak points could also be of use. His weak points are his head and his right shin. When you successfully shoot a weak point, you’ll dramatically decrease his stamina, and if his whole bar is depleted, you can get in a powerful Stun Attack.
When you’ve defeated Callin in combat, remember to confirm the kill to complete this Zealot boss fight. You’ll earn a random powerful rune for this fight and a few other supplies.