Hard West Ultimate Edition

Usable items in game have a round, potion vial symbol next to them in your inventory. You can carry 2 of them at a time into battle. Simply use 25 different items throughout your gameplay for this. Achievement is stackable, so you do not have to do this all in a single scenario., nor all on a single character.
So far I have found 27 different usable items:
#1 - Relic
#2 - Tobacco
#3 - Bottle of Liquor
#4 - Mandrake Root
#5 - Coca Leaves
#6 - Stun Bomb
#7 - Healing Elixir
#8 - Noise Bomb
#9 - Shaman's Pipe
#10 - Snuff
#11 - Gas Bomb
#12 - Eagle Elixir
#13 - Dose of Opium
#14 - Dried Healing Herbs
#15 - Dried Meat
#16 - Meteor Dust Bomb
#17 - Blood Berry
#18 - Ghost Town Fruit
#19 - Nail Bomb
#20 - Vitality Elixir
#21 - Mandrake Powder
#22 - Ginseng
#23 - Dexterity Elixir
#24 - Petrification Bolt
#25 - Stimulant
#26 - Medical Bag
#27 - Angelic Sphere

Some items are found through gameplay, other items are unlocked by specific actions or missions. I do not think this would come naturally with gameplay though, as I rarely needed to use any items. I suggest keeping track of what you use, and on missions before you kill the last enemy, use an item you still need just for credit towards this. On the final scenario I bought 3 items from merchants I had not used yet just to finish this off.
I will update my list if I find anymore items that can be used.

Gratz on your chievo!