Rather than being hit or miss for having ~25% less defense and ~50% less attack, I believe Poison should be implemented in stacks. Each Poison attack would increment the amount of Poison Status an entity has by a set amount. The amount of Poison stacks equates to the degree of poison effects.

Poison status has a base time of 3 seconds, which is increased by half of the Poison stacks on an enemy (up to 30 stacks), and then beyond 30 stacks, the square root of the excess stacks is added to the duration. After 30 stacks, poison is not any more effective and gains less duration.

EDIT: What if instead of Poison stacks affecting over a duration calculated from the amount of stacks, Poison stacks effectively affect entities up to 30 stacks, and get removed over time? Say, remove two poison stacks per second?

The effects of Poison will be linear.
At 0 stacks, healing has 100% efficiency
As the stacks approach 10, healing deals 0% damage, attack reduced by 20%, defense reduced by 10%
As the stacks approach 20, healing deals 100% damage, attack reduced by 40%, defense reduced by 20%
As the stacks approach 30, healing deals 200% damage, attack reduced by 60%, defense reduced by 30%
i.e.: At 25 stacks, healing does 150% damage, attack reduced by 50%, defense reduced by 25%

Swords:
Vile Striker: 2 per regular hit, 5 per charge attack hit
Dread Venom Striker: 3 per regular hit, 10 per charge attack hit
Obsidian Edge: 10 per charge attack/tentacle hit

Guns:
Obsidian Carbine: 2 per bullet, 10 per tentacle hit
Toxic Catalyzer: 1 per bullet, 5 per charge attack explosion
Virulent Catalyzer: 2 per bullet, 10 per charge attack explosion
Biohazard: 3 per bullet, 15 per charge attack explosion
Toxic Needle: 1 per bullet, 1 per charge bullet
Blight Needle: 1 per bullet, 2 per charge bullet
Plague Needle: 1 per regular hit, 3 per charge bullet

Bombs:
Toxic Vaporizer: .1 per tick
Toxic Vaporizer MKII: .2 per tick
Toxic Atomizer: .4 per tick
Venom Veiler: .5 per tick
Obsidian Crusher: 10 per explosion

These are the raw possible Poison Stacks, with nothing about the poison defense of entities.

Thanks for reading, my brain is fried.