There is very easy guide how to complete “TRESSLER – HUNTING FOR GOLD” mission.
Hunting For Gold Mission Guide
The process of the mission
At the beginning you should find group of animals at least 4. Next you have to kill them in the same area until you get pink/purple colour like in this picture below.
The best tactic is lure 4 Fallow Deers > kill 1 > use decoy > repeat 3 times > done
Congratulations! Hunting For Gold(TRESSLER) missions done.