To get this achievement, you must get the finishing blow on an enemy with your Dynamite ability from 30 meters away by shooting it.
Dynamite is Ashe's second ability, upon use you will toss out the dynamite (kind of floaty at the time of writing this). At the center of the explosion, the damage dealt is about 75, while the damage over time fire will deal about 100 total. It will be easier to get this achievement if the dynamite is shot in the air (especially so if they have a Reinhart or Orisa), but can be shot on the ground for the same effect. If going for the latter, you have 2 seconds after the dynamite lands to shoot it. Depending on your abilities, it may be easier to shoot it in or out of ADS, most will find hip fire easier.
Now the 30 meters is the confusing part, while trying to go for this with friends we all had different ways of doing and different maps. One had very open spot on Route 66, from the Arch to the Train and unlocked it. The second did it on the first point of Gibraltar, from the lower door near the cliffs of the left building to very close to the mini Medkit (the one furthest away from Attacker spawn near the cliffs) and unlocked. The third unlocked it on King's Row, the Ashe was on the upper floor of building to the right of the bus, just barely able to look out the window on the right, they tossed it near the opening of the balcony leading to the alleyway basically in front of the window and shot it in the air, then both the Ashe and the enemy go further away til the enemy died to the burn.
The best way to gauge the 30 meters is to go into the Practice Range, in it there are ranges to shoot at enemy where there are markers for the measurements. The one with the lone bot near the cliff is the best range with it's starting at 0 and 30 where the bot is. Unfortunately this will give you only a general idea of 30m, it won't be easier factoring in terrain.
200 hp heroes need at least a hip fire shot into them before they can be taken down by the dynamite, body or head. 250 hp heroes will require an ADS shot to the body or a hip fire shot to the head before being taken down by the dynamite. Heroes with 300 hp or more (and possibly with armor, have not tested it with armor) will be much harder to secure the kill with the dynamite. The damage to get them close can be done before the dynamite or after the explosion, but it must be done before the burning status is gone.