The spriggan you can kick into the lava can spawn at various locations next to the lava streams in the eastern part of Galen. An NPC called Darroli Ilsene will be calling out to you to kick the spriggan into the lava.
You can walk up to the spriggan and use the syngergy button to kick it into the lava. Even though the spriggan is hostile, it will not attack you when you approach it.
Once you have kicked it into the lava, you will be ambushed by two more spriggans.
That's all you need to do to complete the Firestarter achievement in ESO.
A map of Galen where you can kick a spriggan into the lava
