The monster in question (a shadow dragon from a 3PP adventure) had resistance to all damage - excepting radiant (which accounted for only one attack).3.5 style damage reduction & resistance is your friend as it will apply a flat reduction to every attack & spell by every player.
I roll in the open. It's Initiative stunk. The druid summoned in dire wolves, who got it before it could get fully airborne and dragged it to the ground.Why was the dragon in melee range?
And why did it stay in melee range? It was pinned under a folding boat. There was nothing I could think to do, and I have been DMing since the 1990s. Essentially, I was painted into a corner. The dragon was a loner, in a hard to reach cave (120 ft up a cliff).
Anything I'd do would seem cheap and reek of desperately trying to snatch a victory from these teenagers who were very excited about what was happening.
What I need to do is to set up future encounters so that something this stupid and anti-climactic never happens again. And I think it's obvious that there's too many players to be able to use any of 5e RAW - especially now that they're mid-level (5th level).