To enter Instinct Mode you need to wait until your Instinct Meter (the orange bar under your health bar) is full and then hit HP+HK (RB + RT by default). Your Instinct Meter fills up as you either take damage or pull off Combo Breakers.
Once you're in Instinct Mode you can cause "Chip" damage which is damage caused through an opponents block. Accumulate 10 points of this Chip damage during (I believe) a single Instinct Mode in any fight and the achievement is yours.
If you want to do it using the "so easy it you'll wish you'd done it properly" method then head into Practice Mode.
- Main Menu, Local Play, Practice
- Select Sabrewulf for yourself and any opponent.
- Once practice begins pause the game.
- Press RB to access "Dummy Options" and set Guard to "Block All"
- Press RB to access "Practice Options" and set Instinct Meter to "Infinite"
- Unpause and hit HP+HK to enter Instinct Mode (you gain a blue glow).
- Hit your opponent until the cheevo pops.