Practice, thorn should be the easiest but just have to get used to the kick and thorn is a two hit kill if you shoot the head twice and let the burn kill. Sometimes the burn dosent kill due to the enemy player having a really high armor rating like a titan.
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I've hit someone 2 times head and 1 body with the tick and they not die
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Well either the armor was over the top, the player got some recovery boost from a weapon like red death, suros, or some kind of melee. The other explanation is that you didn't let the poison effect continue and reaplied it after I think the fourth or fifth tic.