Reviewing my demo, this is actually pretty simple as long as you minimize the work the decoys have to do.
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×The guard will kill three golems on his own.
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×After he does, attack the lower-right corner of the second giant from the lower-right. The guard will destroy one golem on his way to the decoy; you now have only three left.
Kill one with a decoy and get the guard to kill the other two on his way out; if you can't work out how to do that, let me know and I'll give you more help.