Hunter's Instinct is a tactics-scaling mutation which reduces the cooldown of skills for each enemy killed with a ranged weapon.
Details[]
Special Effects:
Each enemy killed with player's ranged weapons reduces the cooldown of skills by [0.3 base] seconds.
Scaling: +0.12 seconds per Tactics stat
Number of stats
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20+
...
Mutation effect
0.3s
0.42s
0.54s
0.66s
0.78s
0.9s
1s
1.1s
1.3s
1.4s
1.5s
1.6s
1.7s
1.9s
2s
2.1s
2.2s
2.3s
2.5s
2.5s
...
Notes[]
This mutation triggers if the enemies are killed by attacks from ranged weapons, parried shots (using Shields), and bombs/arrows from weapons' affixes (including Melee Weapons). Does not trigger with melee attacks and skills.
Due to how the game qualifies what counts as a ranged kill, killing an enemy using a melee attack followed by a ranged weapon will not activate this mutation. However, it will activate the mutations Predator and Killer Instinct.
Predator and 
Killer Instinct.