Move to H12.
Use Acrobatic Strike, as part of Acrobatic Strike shift 1 square before making the attack.
Shift to G11. Now I'm flanking.
Attack +3 (flanking) vs. AC 1d20+12 : 7 + 12, total 19
Damage 1d4+5 : 2 + 5, total 7
Sneak Attack Damage 2d8+2 : 5, 1 + 2, total 8