This is a situation—“big” hit, low output—that I definitely don’t fudge. The dice have spoken! I handle it 100% narratively instead.
When the crit is rolled and the instant table excitement dies down, I immediately call for the damage roll. When the total output is disappointingly low, I let the storytelling take over.
I describe the epic precision and strength of the character as the seemingly devastating strike lands. Depending upon the exact nature of both the weapon and the foe, I usually describe how the blow does some kind of visible damage and/or leaves a gruesome wound or mark.
Then I describe in equally epic terms how, despite having just been tagged in the worst of ways, the target powers through the demolishing hit and miraculously stays standing. Maybe the hit drew first blood on a creature that is deceptively tough, so the blow that would have put down a lesser monster suddenly reveals this opponent’s true strength. Maybe the battle has been viscous and prolonged, putting the opponent at low hit points, and the big hit that the player hoped might have been the coup de grace turns out to be the strike that fuels this pitiful “walking dead monster” in its will to live a few moments longer and make an epic last stand.
There are as many possibilities for telling the story of moments like these as there are combinations of die rolls, monsters, and player characters. Tell one that your players remember, whether they stand fast and eventually defeat the foe who would not go down, or whether they suffer unexpected tragedy as fate turns on them suddenly when they do their best and still lose the day.
This is not a time to play softball—particularly in a system like ICRPG that has concrete mechanics like Hero Coins (and Memory Rings and Dark Pact) baked into the system to turn such letdowns right back around in favor of the adventurers. This is not the loss of a beloved PC that the player has invested in heavily—at least not yet. This is just one bad die roll with the promise of many more to come in one’s time at the table. This moment of unexpected disappointment is a vital part of the game, reminding them that few things in the dungeon are absolutely certain. That’s what makes it an adventure.
If you have players who still whine about it after more than a minute, you are only doing them a favor and fulfilling their need to have someone help them put on their big-person pants one leg at a time and learn to remember that, while we play games for fun and the session should leave everyone better off at the end of the night, that fun is born from the rhythm of individual highs and lows, of which this low damage roll is only one. Learning that lesson in-game only reinforces its truth and value in our larger lives.