two level two lightning spells work for taking out a level 3 mortar, but you will need either 3 level 2 lightning spells or 2 level 3 lightning spells for taking out a level four mortar. For attacking town hall 8+ players, or level 5 mortar players, you might need more lightning spells and will need higher leveled lightning spells
If you have extra space in army camps use more giants and/or wall breakers. Unless you are town hall 9+ you will not need any more barbarians or archers.
(No execution intro)
Start off by deploying both your lightning spells on one of the mortars. This should take it out.
Deploy your giants. You should only deploy them on one side of the base, as they will then work together, and not get too bunched up nor spread out.
Deploy about 5 wall breakers behind the giants. Try to time it right, deploy them right when a mortar blast hits so you don't need to be worried about that. You should also try to time it so they don't come 100% simultaneously, as this will probably allow them to break through one or two of the inner compartments as well. You want to start deploying right when the giants have reached the wall, to ensure that the giants are taking the hits from the point defenses, not the wall breakers.
Deploy all your barbarians in a circle around the base. The way to do this is by holding down and just sliding your finger around the screen in a circle.
Deploy the remainder of the wall breakers on the side where the barbarians seem to be getting on best
Deploy all your archers in the same manner you did barbarians.