I might have the full attack with all 3 attacks, roll 1 die less per attack. A 9 die-piercing two, 5+ rot, 7+ rot/bleed is a bit much. Making it 6 die makes sense to me.
^This works too. But I was also thinking that 2 armor 15 life with that 9 dice total attack was definitely too much for a 17 mana level 4 creature. Darkfenne Hydra is level 4 and costs 16 mana and has a 3 dice triplestrike attack with no effects, and only 1 armor instead of 2. It does have regen 2 though. So for only 1 more mana and 2 more spell points (if playing wizard), you're trading regen 2 for +1 armor, charge +2, piercing +1, and rot chance. Lowering the dice of the attack would be a viable solution, or increase the casting cost and level. One notes that enchanting it will make it even more survivable than it already is. While 2 armor and 15 life might not be enough for a high level high casting cost creature to stick around long enough to pay itself back, you can obviously still enchant it, or attack and destroy things that threaten it, which it seems designed to do at any rate. Cast eagle wings on it and it will be a beast.