Statting out Hellcat: Soulbound Ballista Master

[Escheton]

Hey, for a City of the SpiderQueen game I had in mind making a mountainplate wearing half-hobgoblin (no stat adjustment, darkvision, +2 move silently, fave class any) with a soulbound ballista.

•Starting level: ECL 10
•Starting gold: standard DMG
•Ability scores: roll 24d6; assign dice to ability scores afterwards (3/ability score, for all abilities).
•Available library: DMG, MM, PH, XPH, Complete series (except complete psionic), MIC, SC, ToB and any Forgotten Realms sourcebook


Help me make this guy awesome.

So far I have 2 lvls of Psiwarrior for the soulbound weapon. The rest is up for grabs.
Ballista don't use your base attack so a low bab class would work.
I am personally thinking psion with practised manifester and taking call weaponry as a psion with a feat.
With the +1 enhancement aptitude I am taking a great crossbow and hand-crossbows for the same exotic weapon proficiency.
Perhaps a exotic melee weapon as well.

Stuff like exploding ballista bolts and such I shall also be taking.
So a list of ammunition-suggestions will be greatly appreciated.

[Eldan]

Correct me if I'm wrong, but: doesn't that race suck?

[Escheton]

Totally, its worse then half-elves.
But I like the flavor of hobgoblins, but they are not worth the +1

[Fax Celestis]

Heroes of Battle has a bunch of feats you'll be interested in. Ballista Proficiency, Expert Siege Engineer, possibly Plunging Shot or Ready Shot...Don't forget that a ballista is a really really really big crossbow, so theoretically you could apply crossbow feats to it.

[Escheton]

Right, such as Hand Crossbow Focus:( DotUD Reload any Crossbow (or other weapon that needs to be reloaded) as a free Action, and get a Weapon focus to boot.)
Well, if I manifest it as +2 aptitude
Which I will do most of the time anyways.
As such I am considering taking a warblade lvl to change the base EWP to the the handcrossbows.That way the great crossbow (2d8, 18-20/x2) will also be reloaded as a free action if it's +1 aptitude. Which it will be.

Mountainplate and zen archery combined give me a decent ranged attackroll without needing much dex.

The main issue is: how does one up the attackroll of the ballista?
Checked the dmg and you just get a -4 to the roll because its basicly a huge heavy crossbow. I recall it being a flat d20 roll. Anyone happen to know where I get that from?