Actually, this is kind of OP.
Before HP buffs:
[1x Card] Trojan Horse [5 + 3 ] for 6 poison
[3x Cards] 3x Poison [6 ] for same effect.
[2x Cards] Forest Scorpion [3 ] + Adrenaline [6 ] for same counters.
Other effects take at least three turns.
To expand on this
Forest Scorpion:
1|2 (attack) +0 (2hp) +3 (venom x1) +0|-1 (-1 or -2 upgraded) = 3
+1card
Trojan Horse (suboptimal usage)
-2|-3 (attack) +0|1 (3|6hp) + 12|18 (venom x4|x6) +0|-2 (-1 to -2 upgraded) -1 (duo) -1 (high activation cost) = 12-4|18-7 = 8|11 = 7
|10
+ 1card
Trojan Horse (regular usage 1 Blessing | 1 Chaos Power)
-5|-8 (attack) +1|2 (6|9hp) + 30|36 (venom x10|x12) +0|-2 (-1 to -2 upgraded) -2 (trio) -1 (high activation cost) -3|-3 (cost of additional card and upgrade) = 30-10|36-14 = 20|22 = 19
|21
+ 1card
Summary: The card's cost needs to be absurdly high if you want to keep the Venom at Attack x2.
If the effect only did 1 poison per damage dealt
Trojan Horse (regular usage 1 Blessing | 1 Chaos Power)
-5|-8 (attack) +1|2 (6|9hp) + 15|18 (venom x10|x12) +0|-2 (-1 to -2 upgraded) -2 (trio) -1 (high activation cost) -3|-3 (cost of additional card and upgrade) = 15-10|18-14 = 5|4 = 4
|3
+ 1card