Keep in mind that all classes are not created equal. A Wizard 10 presents a vastly more powerful challenge than something like a Truenamer 10 (especially since the Truenamer's primary mechanic is nonfunctional due to poor design).
The first thing you should decide is what sort of foe you want this boss to be. Next, build it at least two levels higher than the average party level. After that you should compare your Boss to a creature of the "same" CR; is your level 11 Fighter on par with an
Elder Earth Elemental? You may need to add a level or three to make your Boss an appropriate challenge, but this largely depends on your party's abilities.
If you have a party of all melee-focused beatsticks, an airborne foe has half the battle won already; even a simple
Levitate spell will put your Boss out of range of the deadliest stuff the beatsticks can dish out. Conversely, a party of flying, summoning, invisible spellcasters will require a Boss who is capable of meeting those multiple threats. Perhaps he's a trained mage-killer (with the appropriate feats and gear), or is himself a spellcaster.
An important consideration is timing. Those arcane casters need their beauty sleep if they're going to be casting anything at all, and if he has his minions interrupting them during the night before making a personal appearance, he can give the other players a chance to shine.
Lastly, consider tactics. If the party surprises the Boss, even a large CR split might not make him challenging. If the Boss catches the party while they're sleeping or otherwise vulnerable, it can drastically alter things in his favor. Cover, concealment, traps, terrain, and a wide variety of other factors can contribute to a challenging and memorable encounter if used properly.
A good idea is to playtest your potential Boss by using copies of the PCs. This is particularly easy on the Weave, and not particularly difficult in person. Run through the likely scenarios once or twice, and ask yourself whether or not the Boss is a satisfying challenge. Tweak as needed and repeat until you feel comfortable with the result.