The Dis's page

Simple one, how many negative hitpoints before a familiar dies?
The rules appear to specifically call out that they're a normal animal which would be -10hp. However another player insists that like a character it's to -con because they count as a magical beast for 'effects'.
What an effect is is not defined anywhere that I can see.

On page 91 (below the climbing DC check table) of the core rulebook it states that when climbing arcane spells can be cast, the caster simply clings on with only one hand. It makes no mention of any penalty for this. So to confirm this is one hand that can make no somatic contribution to the casting of a spell and yet has no penalty.
Why then do shields/bucklers and using the optional rules for armour parts of armour on only one arm incur an arcane spell failure chance? The hand under these circumstances is still available for some contribution.
Seems utterly backwards to me.

An issue came up in a recent adventure session whereby I as a ranger managed to hit an ancient red shadow dragon with a greater arrow of dragon slaying only to be told that it was immune to any effects the arrow might have had...Needless to say we as a group had a nice long discussion about it and I accepted the GMs arguement that tough, I did no damage as the arrow needed to be attuned to undead slaying. (I killed the wyrm in the next round anyway).

Does this seem correct as I still find it strange that a dragon is immune to arrows of dragon slaying.