Warhammer 40,000 FAQ CORE BOOK Indomitus Version 1.2 wrote:*Pages 361-362 – Rare Rules, Always Fight First/Last
Add the following paragraph to the end of this Rare Rules entry: ‘Note that the Counter-offensive Stratagem requires you to select a unit that is eligible to fight. This means that if a unit is under the effects of a rule that says that it is not eligible to fight until after all other eligible units have done so, you will not be able to select that unit to use the Counter-offensive Stratagem.’
Note that
eligible is the key word here. The newer versions of Fight Last rules use
not eligible in their text. Older rules use
cannot be selected, which means they are still eligible. In those cases, you can use Counter-offensive to jump in and fight.