Did this today so finish my season journey and accidentally found a pretty good tip that I didn't see in any of the youtube video guides I watched. Cube the handbow that marks enemies you hit for death and then you can see which enemies survived your Impale so you don't hit them again. The shield guards will usually survive because of their block ability.
Two more tips:
-Chemical Burn rune will let your single impale finish off those shield guards
-Avoid using proc effects on gear that deals damage like sash of knives. It will kill enemies too easily for you to land impales