You're overrating Kurou's reluctance to use Demonbane, though... I figure that if Kurou tries to attack at full power and it completely fails, he'd summon Demonbane. And if Laharl started summoning a meteor or doing something suitably ridiculous, he'd summon Demonbane. If Laharl blitzes him and he gets an out with Mirror of Nitocris or Atlach-Natla, he'd summon Demonbane. The only case where he doesn't is if he is unable to react to the counter, and I don't think Laharl is really all that fast.
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"As the size of an explosion increases, the number of social situations it is incapable of solving approaches zero." -Vaarsuvius, Order of the Stick