Animated ballista on wheels? What CR is it?

[Kafana]

Ok, so I'm looking into having a battle with an animated ballista on a cart. The ballista itself should be set up so that it can automatically be reloaded with a full round actions. Now, the idea is that every even round the ballista fires a bolt, doing 3d6 damage, and every odd round it reloads while doing it's normal attack (slam or trample, depending on size). I'm guessing the ballista with the cart should most likely be a large animated object, but perhaps it should be huge?

Anyway, what EL would you give to this encounter?

[VoxRationis]

I suppose that would depend a lot on the attack bonus. By itself, though, it wouldn't have too great a tactical ability (an Int of -- sort of cuts into that) and couldn't choose good targets, nor do the kiting it really needs to in order to be effective.
Still, 3d6 damage a shot... A Large animated object has a +2 BAB, no Dex bonus, and 52 hit points (mostly being made of wood, it has hardness 5), and is natively CR 3. What you're suggesting exchanges a +5 attack bonus with 1d8+4 damage for a +2 attack bonus with 3d6 damage, and wouldn't be smart enough to focus on lightly armored targets. 3d6, with an average of 11 damage, can whittle down a fighter in a number of shots about equal to the fighter's level (if we assume they roll reasonably and have a good Constitution score), maybe a round earlier, if we assume it hits. However, the fighter easily has an Armor Class of at least 17 at level 3 or 4, depending on the exact equipment, so the ballista will only hit 30% of the time. If we assume that the group can do 15 damage a round, on average (this is definitely a low-level CR, even if it's not 3 precisely), it'll take 4 rounds to kill it, during which time the fighter will have taken 1-2 shots for around 21 damage, depending on rolls. I'd say that's balanced enough for CR 4.

What does everyone else think? Is 15 damage/round a reasonably assumption against AC 14 and hardness 5?

[Lightlawbliss]

I would have no problem calling it a CR 3 personally. One fact Vox seems to have overlooked is that the ranged attacks would provoke an AOO, and since every other round is spent slamming: it will likely be in melee range rather quickly. I would actually consider the ranged attack a net nerf with your uses it every other round part.

[Anachronity]

Yeah, it would have to be intelligent to be a real threat. If that's what you're looking for then I would recommend the Fiend of Possession prestige class from the Fiend Folio. It's 3.0 and very OP, but it's also very flavorful (and you're the DM!). Alternatively you could just have someone directing it.

If it were intelligent or directed it would probably be closer to CR 5. Since it would likely have wheels, it is also important to know what its move speed would be.

[Gavinfoxx]

*whistles*

I'm just going to leave this here..

http://www.giantitp.com/forums/showthread.php?t=271165

Rather than what you said, my option is made to be actually useful at the point you get 'craft construct', and remain useful, and have a relevant strategic use...

[VoxRationis]

Good point; I was just ignoring the slam generally.