Make getting to the exit your priority over killing all the wraithwings.
Once you're at the exit, there are two ways you can kill the remaining wraithwings, as explained in the other hints above. To review, they are:
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×Stand such that the remaining wraithwings hover over bridge. The briar will kill them for you.
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×Or, if you can move far enough east such that the wraithwings line up directly to your west, you can move west and kill them with your sword against the growing briar.
Keep trying!