(1) You are using a UGL.
(2) Marines are not pushing.
(3) Xeno(s) just dashed at a marine gunline.
(4) You launch the HEFA at the xeno(s) line of retreat.
(5) You are absolutely certain that nothing will block the path of the HEFA.
Why?
HEFAs are inefficient as a weapon, but can still be used for crowd control.
HEFAs will never kill a xeno on its own, nor do they have the massive stun potential of HEDPs. But even though it has a small stun radius, it still stuns xenos. Even though its damage is terrible any distance from the grenade, standing on a HEFA still hurts xenos.
Correctly timed, a HEFA launched in this manner will force the xeno(s) to either:
(a) Attempt to retreat through the HEFA blast, risking a damage burst and/or being stunned while under fire from marines.
(b) Delay their retreat for the HEFA to detonate, extending the time they are exposed to marine fire.
(c) Move around the HEFA, also extending their exposure to marine fire.
Reasoning for these Conditions:
(1) UGL gives you much greater control over HEFAs. Nothing is worse than priming a HEFA in-hand, only to get knocked down or stunned by xenos or FF.
(2) If you attempt to cut off a xeno with a HEFA while marines are pushing, you exponentially increase the risk of FF.
(3) The goal is to ensure that no matter how the xeno(s) act, they experience a negative outcome. To achieve this, the xeno(s) must be in a position where they are pressured to escape from marine fire.
(4) Your goal isn't to directly hurt the xeno(s), it is to limit their choice and movement.
(5) HEFA has a reputation for a reason. If you don't know, don't blow.
Spoiler: