Ok, I'm a DM and I'm going to have a "situation". So i went to the rules to preventibly know the answer... and still not sure.
Situation: got a bad sorcerer with greater invisibility who is going to be casting fireballs to a bunch of good adventurers... what if they want to counterspell. Sounds reasonable way to go for me.
COUNTERSPELLING(pag 207): http://paizo.com/pathfinderRPG/prd/magic.html
Spellcraft check (DC 15 + the spell's level) + cast the apropiate spell (fireball in this case) = automatic counterspell. Or you may try dispell magic without spellcraft check (or maybe try this way if you fail to id the spell beeing cast).
QUESTIONS:
1. If bad sorcerer is invisible they just hear arcane words, so that must be a harder Spellcraft check ¿How much?
2. If bad sorcerer is invisible ¿Do they have to pinpoint him "to know where to aim" before atempting a counterspell?
3. If bad sorcerer quickens magic missile or another spell (quicken = no verbal or somatic) ¿Can it be countered at all?
OTHER THINGS I DONT UNDERSTAND AND DONT KNOW IF THERE IS AN ERRATA, DIDNT FIND IT ANYWAY:
Spell-like abilities cannot be used to counterspell, NOT CAN they be counterspelled. (pag 221)
Appendix 1 (pag 554) ... Spell-like abilities CAN BE dispelled and counterspelled as normal
4. ¿Any errata for this i haven't seen? ¿Or does this mean they can be countered with dispell magic but not with the same (or opposite) spell. Ej fireball vs fireball or haste vs slow?
5. It's the same for item creation: 5+CL (Magic Item Creation: pag 548) or 10+CL (Item Creation Feats : pag 112). ¿Which one is correct?
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Spellike Abilities (pag 221)
Spell-Like Abilities: Usually, a spell-like ability works just like the spell of that name. A spell-like ability has no verbal, somatic, or material component, nor does it require a focus. The user activates it mentally. Armor never affects a spell-like ability's use, even if the ability resembles an arcane spell with a somatic component.
A spell-like ability has a casting time of 1 standard action unless noted otherwise in the ability or spell description. In all other ways, a spell-like ability functions just like a spell.
Spell-like abilities are subject to spell resistance and dispel magic. They do not function in areas where magic is suppressed or negated. Spell-like abilities cannot be used to counterspell, nor can they be counterspelled.
If a character class grants a spell-like ability that is not based on an actual spell, the ability's effective spell level is equal to the highest-level class spell the character can cast, and is cast at the class level the ability is granted.
PDT: I'm spanish and this is a big post, I hope i made myself understand and I didn't kick the dictionary too many times with my poor english, any help with this is also welcome ;-)