Hydra

172 average HP
With a party of 5 
You would be able to take this thing down easily IF
No one goes down in the second round and Everyone does atleast 25 damage

@KaptainKittens

I animated a skeletal one and did this. By the time I reached the final boss 18 sessions later, my skeletal hydra (created with Animate Dead) had 630 heads. That's an average of 6615 damage per round if all attacks hit. The final boss was a tarrasque. It did not survive the Hydra's turn.

 *cough cough Frost Brand cough cough*

 Hai Hydra! Hail Hydra! Hail Hydra!

Probably like 30 because anymore and it's neck would break from the weight 

We did at level 2 lmao xDDDDDDD

For maximum heads theoretically possible, I did the following absurd scenario:

1v1 (1 player character vs 1 hydra), 25 non-fire damage per turn, hydra has highest health possible ((15×12)+75=255)

Since with every head loss and subsequent dual regen the hydra effectively only loses 5hp per turn, it would have 51 heads by the time it reaches the lethal range of 25hp

freaking awesome

The Hydra if it took exactly 25 damage and you let it regrow its heads would have 35 heads in total and 22 life left this is using its average 172 hp which loses 25 hp and heals 20 because it heals 10 hp per head which is two heads. Assuming that it started with five heads because it says so in the trait if it took 25 damage. That is 172-5x30={-25+20) = 22 hit points we stop here because another hit would drop it to 0 before it could regenerate we can argue about whether it can regrow heads and hp while unconscious but i am going to end it there. So with 30 regrown heads each equaling 2 in its place plus the starting 5 heads you end with 35 total heads and 22 hp

35 actually