Debuff the person directly hit by the arrow - they take 1 poison per tick with the debuff - this lasts 2 seconds (exactly as long as the poison lingers for), meaning that the bow will not one shot most guardians, and the damage will be about in line with Wishender - Projected damage output of 160 (152 + 8 ticks of 1 damage)
Increase poison damage in the cloud by 2 per tick (5 poison per tick for those that are in the cloud) - Projected damage output of 40 (8 ticks of 5 damage) should the poisoned guardian not be struck by a second Le Monarque arrow.
This means that you'll be able to deal up to 200 damage over 2 shots on 2 enemies if you time them right - the caveat being that you'll lose out on damage if you don't wait for the last tick of poison from your first shot.
If I'm right on how much health + shield you have with each resilience tier - if you hit the second shot just after the 6th poison tick, you'll kill guardians 3rd tier resil and lower (190), after the 7th you'll kill at 6th tier resil and lower (195) and you'll kill all but maximum resilience guardians if you time your shots perfectly (200).
This takes away the main current issue of one shot lemon, but maintains the original purpose of the bow
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Or just make it so the poison can't deal the killing blow, that'd probably fix most of the complaint with it