Stand on a rooftop while on fire, directly in front of heavily shielded enemies. Wait for them to throw their spears, then use the ability that lets you reflect projectiles to send the spears back at them. This will kill them while you’re burning, which is key to completing the achievement.
PS: For this to work, the enemies need to have a red skull icon next to their name. If you're like me and missed the achievement during your main playthrough and are now in the endgame chasing trophies, just raise the enemy difficulty to "Nightmarish" or similar so that enemies have red skulls again.