The two dragons, for some reason, are incompatible and cannot be bred together.
Cold and lightning
To date, there is no grass dragon in DragonVale. You may be thinking of the plant dragon. See the related question.
Fire Dragon and Lightning Dragon
Water dragon.
Metal and lightning
Level 11
Reported results of lightning + obsidian are:obsidianfireflyairmotleyspringaquamarinesolar eclipseequinoxmagneticcrystalstorm
Meteor+Lightning & Air+ Copper
Cactus and Ash Dragons.
I got it by using a Lightning and Plant Dragon.
Lightning & Cold Dragon Storm & Sonic Dragon
if you mean lightning you have to buy it, if you mean electric there is no such thing