Most effective damage type against Charr ?

Are Charr naturally weak to any certain damage type ?

No, they're not.

If you take a look at Wiki:
http://wiki.guildwars.com/wiki/Charr_Blademaster
http://wiki.guildwars.com/wiki/Charr_Sentry
http://wiki.guildwars.com/wiki/Charr_Wardkeeper
(Random ones I clicked)
Each class has a different armor rating for certain things.
So no, I don't think overall they are weak to anything in particular.

Ebon Vanguard skills. That's it.

sheer willpower and smart tactics

They take 20% more damage from weapons with max Charrslaying mods.
thats pretty much it.
(note: slayer mods do not cause spells to deal more damage)

maybe someone will quote me on this and type lol but

arent cats afraid of water most of the time :3

Lol

But anyway - there aren't any racial weaknesses in Guild Wars. Except maybe ice elemetals vs. fire and vice versa.

they are naturally weak to my damage +25% vs. charr bow grip and sword pomel

You dont have a Shield of Superior Catnip? Without that you can only deal normal damage and take normal damage from them.