That is very, very bad with giant lizards. The old monster manual gave a special ability to these, saying that if a lizard hit with a 20, it caused double-damage. Since a 20 causes double damage anyway, I used to say that it caused four times damage . . . but that is when I was still using the monster manual's damage of 1-8 for a giant lizard's bite.
Since, I'd decided that was too low for large creatures and increased it to 2-12. The last time I had a giant lizard attack a party, I did not roll a natural 20. In fact, I haven't rolled a natural 20 with giant lizards for more than a decade, and I did not expect to do it today. But . . .
I'm going to rule that it adds 1d6 to the normal attack and that this is then doubled. I haven't rolled the damage yet, but I expect it to hurt.
The lizard causes ~ oh damn, you're very lucky. I roll a 6 on 3d6; doubled, that is 12. Lucky, lucky you. Enrico is stunned.
Additionally, the creature causes incidental damage half the time to any creature within one hex. Kismet gets off lucky, but Enrico suffers 2 damage from being struck by the creature's body.
The creature then retreats back six hexes, moving between the damaged pillar bases.
The players may now take their move.