This achievement is tied to Agni's passive ability (Combustion). Here's the official description of the ability
*** Spoiler - click to reveal ***After hitting with 4 Basic Attacks, Agni will gain a Buff. On the next cast of Flame Wave or Rain Fire, all enemies hit by those abilities will be additionally set ablaze, taking damage every .5s for 3s.
To get this achievement, follow the steps below:
1.) Perform 4 successful basic attacks. You can track progress on this on the passive UI widget to the left of your health and mana bar. The upper part should fill from bottom to top after each successful hit. Do not get this mixed up with the lower part of the UI widget, which tracks how many shots you get with your Rain Fire Ultimate. Also, the countdown timer is for the Rain Fire ability as well, so ignore this timer for the purposes of this achievement.
2.) Find an enemy god low on health (but not too low). About one bar of health left seems to do the trick if you use flame wave (
) to trigger, and two bars of health if you use Rain Fire (
) to trigger.
3.) Hit an enemy god with either your Flame Wave (
) ability or your Rain Fire ultimate (your
ability). Use the former for those very low on health, and the latter on those who still have a sizable chunk left.
Important: you cannot kill the god with the initial attack, so try to time your attack so that it doesn't finish off the enemy god.4.) Step 3 triggers combustion on the enemy, with damage ticking on them every .5s for 3s (so, 6 ticks total). Hopefully, one of these ticks will finish off the enemy god. If so, achievement unlocked!
If you're having trouble getting the kill on the combustion ticks, try loading up on magical power gear, as the damage done by Combustion is directly tied to magical power. Also, Arena games are ideal for this achievement as enemy gods will clump up, giving you plenty of opportunity to get in combustion kills on fleeing enemies -- the heavy damage of your abilities causes them to run back to base, while their attempted escape prevents your teammates from landing a killing blow while combustion finishes them off.
(Note: This achievement, like all other achievements is SMITE, must be done in PvP matches. Co-op matches vs AI do not count.)