It was intentional. Giants will have to get everything in the second layer before they can get to the core while being attacked by the mortars and the wizard towers.
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I've found that at TH8 you get attacked my TH9s with maxed giants all the time, and when they drop a heal spell on their giants, they are practically invincible to anything TH8s can throw at them (besides spring traps obviously). Lvl 6 wizard towers do epic damage, but against a heal spell (or giant healer) there isn't really much we can do, besides hope the giants funnel into spring traps, or bash on walls long enough for heal spell to run out/healer die, and then let our wizards go to work.