"Radusk
Half-Fiend/Half-Worg
Large-Size Outsider (Chaotic, Evil)
Magical beast 7, Rouge 1
Hit Dice: 7d10+1d6+48
Initiative: +5 (Dex)
Speed: 50 ft.
AC: 18
Attacks: 2 claws +18/13 melee; or bite + 18/13
Damage: Claw 1d6+12; 1d8+12
Face/Reach: 5 ft. by 10 ft./5 ft.
Special Attacks: Spell-like abilities, Sneak Attack
Special Qualities: Poison immunity; acid, cold and fire resistance 20
Saves: Fort +11, Ref +12, Will +4
Abilities: Str 34, Dex 20, Con 22, Int 14, Wis 14, Cha 14
Skills: Yet undecided
Feats: Dodge, Mobility, Scent"
Cool. A spell casting worg could be a fun villian.
Just a few minor problems.
AC is miscalculated too low for the Dex given. Actual AC should be:
10 (base) + 5 (dex) + 3 (increase to size large) + 1 (half-fiend) + 2 (base armor for worg) - 1 (size large) = 20
The attacks line should read:
Attacks: 1 bite +18 melee or 1 bite +18 melee, 2 claws +13 melee.
Gaining claw attacks does not change the primary attack of the Worg to claws (unless you really want it to). Generally speaking, monsters can make one melee attack with each of thier natural weapons, although they do not get iterative attacks with them (normally, slams are usually an exception). One of those attack forms is primary (in this case the bite). The others are secondary, and at -5 to hit unless the creature has multi-attack. Not that with a claw attack, you get one attack with each claw. 2 claws +13 melee means two attacks doing claw damage each at +13 bonus to the attack roll. You wouldn't normally have +2 claws +13/+8 melee, and in any event that notation means 4 attacks and not two - two at +13 and two at +8.
Note that the Worg still has to make a full attack action to get more than one attack per round.
The damage line should read:
Damage: bite (1d8+18) or bite (1d8+12), claw (1d6+6)
The reason? For a primary single attack from a natural weapon(like a Worg's bite before it gets claws), the damage bonus is 1 1/2 times the strength bonus. For secondary attacks, like this wolf's fiendish claws, damage is 1/2 the strength bonus.
Special Attacks should list the Worgs extraordinary Trip ability.
I'd put the CR at 8.
CR: 2 (Base CR of Worg) + 2 (double normal HD, usually works for HD in mid ranges) + 1 (size increase) + 2 (half-fiend template) + 1 (1 level of a combat effective class) = 8
An 8th level party should mow down something (in probably 2.5 rounds) with this low of h.p. and few defences (no SR, no DR) unless you play it right.
EDIT: Doh!
To keep it alive, use darkness to disorient the party, and make grapple attacks (even if it means provoking AoO) to take advantage of your godlike strength (!!). Run whenever you don't have the advantage. Your standard grapple check should do bite damage (!!!). Use the Poison spell to weaken party health over time, and your manueverability to hit a party where (and when) it is weak. Played smart from a position of good ambush, the XP award deserves circumstance bonus. Allowed to be cut down with arrows and disabling spells (Will save) by a party who won the initiative, and you won't even think it worth CR 8.
You do realize how much you increased the Worg's attributes right?
Normal strength for a large half-fiend Worg = 29
Normal dexterity for a large half-fiend Worg = 17
Normal constitution for a large half-fiend Worg = 21
Normal intelligence for a large half-fiend Worg = 10
Normal wisdom for a large half-fiend Worg = 14
Normal charisma for a large half-fiend Worg = 12
So this is really the equivalent of a human with STR 15, DEX 13, CON 11, INT 14, WIS 10, CHR 12. If that is what you were going for, great. If not, the average stats are already quite good. I wouldn't say that the stats are quite worth a CR increase, because you didn't give the wolf wings (from the Half-Fiend template), and class levels usually aren't quite worth (in actuality) a straight 1 for 1 increase in CR unless you increase stats, add magic items, and do other things to match PC advancement.