PRD says:
To cast a spell, you must concentrate. If something interrupts your concentration while you're casting, you must make a concentration check or lose the spell. When you make a concentration check, you roll d20 and add your caster level and the ability score modifier used to determine bonus spells of the same type. Clerics, druids, and rangers add their Wisdom modifier. Bards, paladins, and sorcerers add their Charisma modifier. Finally, wizards add their Intelligence modifier. The more distracting the interruption and the higher the level of the spell you are trying to cast, the higher the DC (see Table: Concentration Check DCs). If you fail the check, you lose the spell just as if you had cast it to no effect.
Basically, when you cast your spell, roll a concentration check. To cast defensively, the DC is 15+double the spell level, so for Cure Light Wounds, it's a DC 17 check. As a bard, your Concentration check is 1d20+Level+Charisma modifier.
If you pass the Concentration check, you don't take an attack of opportunity and you successfully cast the spell. If you fail, you provide an AoO sand lose the spell.
Alternatively, you can stay out of Melee range for a round and cast the spell while holding the charge, then deliver the spell the next round without provoking any attacks of opportunity.
Also, if you were to heal Urthask (currently at -7 HP), but only manage to heal 4 hp, he would stop dying. He'd still be at -3 HP and unconscious, but any sort of magical healing will stabilize him :)
Last edited September 12, 2017 11:33 am