A recurring villain question

[arkangel111]

So my players are about to run into a soon to be recurring villain. In Rise of the runelords, the party had captured several goblins and tortured them, during some commotion one of the maimed goblins got away and has a major grudge against one of the PC's (the torturer). I am debating bringing the goblin back with some levels in the summoner class, more specifically the synthesist archetype. What would you rate a synthesist summoner's CR as? the Level of the goblin is 6th if that helps.

I want to say 6 but that seems wrong as the creature could have an AC higher than the PC's, 5 attacks with haste, all with 15ft reach, and over 100 HP using the Hp transfer ability of the summoner. All of this before I even consider items, or the summoning spell-like abilities.

The 4 Party members include, a 2H PoW Warlord focusing on teamwork abilities (such as flanking), a shocking grasp crit-fisher Magus, a TWF Rogue, and a cleric focusing more on out of combat social situations. The party has exceptional stats (35+pts if done by point system) and standard WBL.

[Abd al-Azrad]

For a technical answer, CR 6. But as is often said on these and other forums, CR is not a perfect predictor of the difficulty of a fight.

IMO your best bet would be to compare the stats of the finished synthesist against some standard monsters of a few CRs around the 7-8 range. Compare its attack bonus, AC, and average damage per round against other mixed-threat monsters with a powerful melee presence, for instance various outsiders or dragons in the younger age categories.