I've been kind of thinking of the whole RPG element system. Anything that you want saved for a specific character, I would use as an instance variable for that sprite.
And anything that would be used multiple times at different layouts/maps, you would set that as a global variable.
By this, you would have multiple variables on one sprite, such as MaxHP, CurrentHP, MaxAtkDmg, etc.
Lets say you want multiplayer, you would want to use a global variable to kill slimes, so that if different players kills a slime almost at the same time, it would increment the same global variable. Its generally how you want to design it.
I don't know how the code is optimized in scirra, but an instance variable would be faster to use than a global variable in most cases.
I hope i am making sense, but that is what i came up with after dabbling in construct 2 for awhile.