Use heroes if available. The Lightning Spells are only needed for tougher levels.
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Use the two Lightning Spells to take out a mortar (if there are any adjacent defenses, try and take them out as well).
Deploy a few Barbarians (or Goblins, if you have extra housing space) to check for traps. Many single player bases are chock-full of traps, especially in gaps in the walls, so make sure to deploy them wisely.
Unleash the Giants and use them as a meat shield to soak up hits. Try to deploy them near the Mortar so they can take it out for the lower leveled troops. If needed to help the Giants get through the walls, deploy some of the Wall Breakers.
Deploy some Barbarians behind the giants in a somewhat spread-out manner. This way, splash-damage defenses such as the Mortar or the Wizard Tower can't take them out.