ok i know that energy resistance blocks per hit and that when it says if something does half fire and half electricity but what about when it says it differently like:
Plasma it is Fire and Electricity.
sure the Starfall Spade in Iron gods is only 2 dmg but is it reduced to half if they have just electric resistance. or must they have both resistances.
I can't speak to the specific example you're citing but...
Take the spell flame strike. That spell does 1d6 per caster level (max 15d6), and half of the damage is fire while the other half is not.
A creature that has fire resistance reduces the portion that is fire.
Let me give you an example. Say the caster is 10th level and rolls 10d6, getting... oh... 36. What that means is that anyone in the spell's area takes 18 points of fire damage and 18 points of "divine" non-fire damage.
Now imagine you've got fire resistance 10. The 18 fire damage is reduced to 8 and the non-fire damage remains 18. So you take 26 damage.
What if you made your save? The 36 damage would be halved, to 18. 9 fire and 9 not-fire. Your fire resistance would absorb all the fire damage, meaning you'd take a total of 9 damage.
If you starfall spade deals 2 damage and half is fire and half is electricity, that should work the same way; 1 and 1. Any fire (or electricity) resistance will negate part.
Basically, do the math. If "plasma" is half fire, half electricity, do the math. Then do the math. Then do the math. Also... do the math. <Grin>
As GM Rednal says, energy resistance and damage resistance don't work the same way.
Damage resistance is expressed as "damage is reduced by X, bypassed by Y". So "DR5/bludgeoning or magic", that means any physical damage the creature takes is reduced by 5 unless the damage comes from a physical source which deals either bludgeoning or has a +1 enhancement bonus (counts as a magical weapon).
Energy resistance isn't expressed that way. "Resist X Y" means that energy damage of type X is reduced by amount Y. If you have resist fire 10, and energy damage of type fire is reduced by 10. With energy, being two energy types isn't a thing. There isn't any mainstream "cold & fire" energy. The closest is the flame strike example I gave. That's a Core spell. Iron Gods is a fairly late adventure path, where writers had been experimenting and continually trying to introduce something new.
In a case like this, the best way to handle this is to refer to prior rules. If plasma = fire and electricity, and something deals 2 plasma, by prior precedent, that means it's dealing 1 fire and 1 electricity. Not 2 "fireandelectricity". A +1 flaming shocking longsword is effectively a [i}+1 plasma longsword[/i] in that you get 1d6 fire and 1d6 electricity damage, or 2d6 plasma. Shrug.
Also, just because... reviewing the item in question, it turns out that the way it's worded IS literally what we're telling you. I quote...
"The Technic League produces these unusual weapons as signature tools for the guards of Starfall and the Palace of Fallen Stars. A Starfall spade is a +1/+1 monk’s spadeUE and has a capacity of 10 charges. When the weapon strikes a foe, it automatically consumes a charge and deals an additional 2 points of plasma damage (1 electricity, 1 fire) on that hit. Attack that miss don’t consume charges. A crystal button on the shaft allows for a secondary attack: a beam of plasma that fires from the forked tip of the spade. This beam is a ranged touch attack that deals 4d6 points of plasma damage (half electricity, half fire) on a hit—it has a maximum range of 40 feet with no range increment. Firing this plasma beam consumes 10 charges."
Hopefully this helps.