This will come with time, although there are some methods to help increase your chances. You can level your hero a little bit, and wait for some enemies to pile up on the battlefield. Then spawn your hero, and use your powerful attack (), trying to kill all the enemies as quickly as possible.
Another method would be to equip and upgrade the Tri-Stone projectile. When you get a large amount of enemies on the battlefield, aim low and explode this projectile early, hopefully killing at least 10 total enemy troops. You can finish off any remaining enemies using different projectiles.
Either way, as you kill enemies, you’ll see the killing streak counter show up on the right hand side, once it reaches 10, you will unlock the achievement.