All spells that affect your own units affect all of them at once no matter where you cast them, the targeting thing for those is more of a confirmation.
@ratchet500 From what I can tell, a giant spawns when the enemy castle reaches 75%, 50% and 25% (when you hear the horn). There's also a giant that spawns after like two minutes. So.. just wait until the timed giant spawns, kill him, then back off when each of the other giants spawn and kill them before resuming attacks on the castle.
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