I say dramatically reduce impact but increase the dot duration. Exploit the exotic part of the weapon. It will still be a two shot, but the ttk for the two shoot is much longer for you have to hit the player again to make the final blow. This could balance it out as tactical hunting tool. Leaving a feared weapon yet, but not completely overpowered.
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Decrease the power of the dot, same time. Say 5 damage over 7 ticks. 5*7=35 damage, not that much damage but still could kill somebody, if they are lucky.
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I was thinking 30 for head shot but 7 hp over 12 ticks. 30 + 84= 114 hp overall. Still a two shot, but you at least have time to counter act. Also leaves a tough choice of continue to engage or run for the thorn user knowing they have to get at least 4 head shots to end them there.