Most classes are not tied into roles, even if they do some things better than others.
The summoner as an example does combat, and it buffs.
Even refining the classes to roles as broad as "Combat" and "Spellcasting" doesn't quite do them justice.
In-Combat, I can think of AT LEAST 3 continuums that classes operate on:
magical<-->mundane
damage<-->control
proactive<-->reactive
Each of the classes would place somewhere in the middle of that 3-dimensional space
For example:
The usual 2-Handed Barbarian build would be pretty much as close to Mundane/Damage/Proactive as you can get.
A similar Fighter build would be slightly closer to the control side and the reactive side (but not over the halfway point)
An evocation wizard would be just about as close as you could get to Magical/Damage/Proactive
But a Healing Buffing cleric would be somewhere in the middle of Proactive/Reactive and Damage/Control
Note how I specified the builds above, instead of just calling out the classes. How a character is built (and played!) has almost* everything to do with what roles they fulfill.
* Also what other characters are in the party.
Alchemists can be pretty versatile. He can really jump around all over the continuum with ease (based on the different builds). Usually, though, very few characters can sufficiently fulfill many roles at once with the same build. However some classes do it better than others, such as Bards & Rogues.
P.S. Sorry for the tangent, probably not helpful anyway.
As the others have said, there's no clear answer based on class. Your best bet is to pick a role and then build the class to fulfill that role. If that role is what the monster was good at before, it's key. If the role is something new for it, it's not.
In case anyone is thinking I'm somehow breaking PCs down to roles (like d&d4e), this is specifically for calculating the CR of monsters with class levels using the system in the Bestiary. This means that there are clear answers for the classes in the Core Rulebook, so there's not really a good reason that there are not clear answers for the other base classes.
In that case, as mentioned above, you will have to base it on the build of the character, rather than the class.
Alchemist could be Spell/Skill or Spell/CombatSummoner could be Spell or Spell/Combat (Synthesist Summoner)
I think it was pretty well answered the first time around. It depends on build of the class, and what the monster was good at before.
If you want a specific example, tell us what monster and what build of which class.
