I think there's a bit of confusion in the book on Daemonic and Unnatural toughness, the trait description for Daemonic say that it should have a number representing the toughness multiplier, but this is not the case for the DP, instead it has Unnatural Toughness(2). Not 100% sure how to interpret that (if they stack or not).
The book says Daemonic and Unnatural Toughness stack, but you're spot on about how the Daemonic trait is missing it's parens. I just interpreted it as UT (2) and Daemonic (1) as his toughness bonus is listed in his stat block as 12 with his trait being 45 (4 for toughness, 4 for unnatural, and 4 for daemonic = 12), but I've yet to see anything where Daemonic is listed with a number in parens, which is kind of annoying.
Also, is Zeal considered a Holy attack? The DPs also have "The Stuff of Nightmares" so they're immune to criticals (save for the ones that kill outright), essentially giving them about 6 or so extra wounds. Nevermind it's Dodge +10 that it could've used (also remember dodge is basic, so you can use it untrained at half your attribute), or if it carried something other than a giant two handed sword that was unwieldy (parhaps two swords given his two weapon wielder and amidexterous traits, or bolt pistols given his astartes weapon training), and the ability to spend fate points (thus potentially surviving any killing blow anyhow, or if he's a CSM turned into a prince perhaps using a chaos version of heroic sacrifice).
I was going to stop there, but this doesn't add up and is nagging me more and more. How many attacks did this BT get? Dealing 24+10=34, assuming he rolls 10's with each attack (average out the chance for RF), means each hit is doing 28 damage (34 - 6 from armor, no toughness). That means at least three hits to bring the guy down, if not 4; 28x2 is only 56, leaving the DP with 24 wounds, the next doing another 28 bringing him to -4, which means with stuff of nightmares he woulnd't really be phased that bad, giving him the ability to fight back. Also, where were his regenerating hordes and CSM allies?
But however you slice it I do tend to agree with Alex here, if you don't kill it in 1-2 rounds you'll end up with dead/crippled players or fate points burned- 85% chance to hit, 2d10+25 pen 6 felling (I assume felling 1?) means your marine probably has a soak about 8 (6 if it doesn't hit the body), meaning you have to roll two threes on 2d10 to bring most players to zero (again nevermind that they get RF due to touched by the fates). Roll better and you can kill your marines outright. That's not even counting the fact that he gets multiple attacks (at least 3 of them, even on a charge), has a +2 to damage due to crushing blow and a +4 to your crit damage due to crippling strike.