Except for Hadros and Hydra Boa (for Round 1 only in the latter's case), the Apex Bosses that do attack on the first turn of every Round don't have Minions with counter-attacks of their own, while the Apex Bosses that don't attack on the first turn of each round (including Imperatosuchus) do have counter-attacking Minions.
But, that doesn't matter anyway. None of the Minions have rending Resistance, so a 60% Rend buffed by Roar is still going to do almost, if not completely, one shot both Minions before they can do anything.
And if their health is low enough that they can be brought down by a single counter-attack from the Boss, then odds are, again, the team just wasn't up to it anyway.