To deconstruct this one bit at a time...
I have two ballistae and an endless arsenal of steel arrows for them.
--You need silver bolts instead. Projectile damage is based on weight. Steel is very light, and makes a bad projectile. That's why bullets are made of lead, because it's cheap and heavy, instead of something light like aluminum, and why depleted uranium is the holy grail of projectile.
--Also, make sure they're good quality. The quality of the ballista parts used determines the accuracy and rate of fire, the ammo quality determines accuracy, and the dwarf's skill level determines accuracy and rate of fire. Also be aware that siege operators are civilians, and will run away from a groundhog into a spike pit. If you have any blind peasants, they won't flee because they can't see. Otherwise, put them back a bit. Ballistae have range, very good range, very likely longer range than any tunnel you would normally build.
I have 20 or so war grizzlies.
--I'm ok with this. A few more wouldn't hurt, to be sure.
A military of 54 well trained, all-steel dwarves (some will be upgraded to adamantine as I manage to score it).
--Hopefully no steel hammer? Refer to above, when considering kinetic weapons. The strength of a hammer or mace, is in its striking power. The more weight it can achieve, the better the strike, much like a wrecking ball. Steel, being light, is a poor wrecking ball. Silver, being heavy, is perfect. Refer to The Beetles' song "Maxwell's Silver Hammer" - That would have been a very heavy, very lethal hammer. Otherwise, steel for armor and anything bladed. Also, NEVER EVER FOR NEVER adamantine hammers! Except for justice use, because blue hammers are less than lethal.
I have a 2x10(x10) steel spike pit linked to a lever.
--1x20(x10) would be preferred. As it is, you have 10 traps the enemy will hit. If you stretch it and snake the route, you can make them run over 20 traps instead. This is clearly twice as deadly.
I have three masterpiece steel floodgates blocking each level of defense/offense for slowing them down and, primarily, containing pathogens.
--Good, except, those masterpieces are someone's lovechild. When they get smashed by demons, the creator isn't going to be happy. You may very well have a tantrum spiral from masterpiece destruction, of all things.
And finally, a dwarf washer at the entrance to the gauntlet to hell.
--This is good, but not totally needed. As I understand it, demons will have syndromes, but not as often as your average FB. Still, never hurts to be over-prepared!
Think i'm ready for the party?
--Clearly, you are not. If you were ready, you'd know that you're never ready. You're simply "less naked". The simple truth is, you are not ready, you never will be, and you will die. Now, let's see how well you can screw over hell before you're sent there!