Here is the full list of all 36 Garshasp: The Monster Slayer achievements.
Execute the first counter attack
Kill 300 Enemies
Kill both Viamboors
Execute the first rage attack
Slay Akatash
Upgrade the sword
Kill 100 Enemies
Find the first Hoom seed
Kill Viamboor
Kill Gandarava, the Sea Monster
Kill Hitasp
Kill Khishma
Kill Viamboor without getting hit
Kill both Viamboor without getting hit
Kill Gandarava without getting hit
Kill Hitasp without getting hit
Achieve the dragon mace
Fully upgrade the sword
Upgrade the mace
Fully upgrade the mace
Fully upgrade the sword and the mace
Find the fourth Hoom seed
Find the sixteenth Hoom seed
Execute 100 counter attacks
Execute 100 rage attacks
Throw ten enemies in the water of the lake while on the raft. (with a splash!)
Do not get hit while sliding down in Siavoshgard
Finish the game in easy mode
Finish the game in normal mode
Finish the game in hard mode
Finish the game with no game overs
Finish the game without saving
Finish the game without using any health fountains
Hit 7 enemies with a rage attack
Do not get hit while sliding down in Jungle
Do not get hit while sliding down in Ooshidaran