4th edition adventure with a young dragon

Beware the wall of text below! Here is a TLDR for those of you who don't feel like reading a mini-novel:

TLDR: Small group of level 7 adventures, most blow all dailys to kill a displacement beast. Decide to go after nearby dragon anyway, dragon rolls terribly and is thoroughly owned. Overall excellent and fun session for the players. Dragon was easier than displacement beast.

First off, DM decides about half-way through to not use a screen because "it shouldn't really matter" [This will matter to the story later]. We were playing a very casual battle-oriented game, no real RP. [DM and I play a more RP oriented game every weekend, but this was with co-workers]

Our characters [rolled up by DM, but chosen by us] include two paladins, a cleric, and a warlock. One paladin and the cleric are human, other paladin is dragonborn, I am the warlock and am a Drow.

We trek up the side of a mountain and get near the summit. The summit has a Caldera and we are near the edge of it. We all roll a perception check. We mostly get relatively low rolls, but the Cleric rolls a 1 and the Dragonborn Paladin rolls an 18. The cleric hallucinates a "flock" of dragons and runs screaming down the mountainside. However, story changes here as Human Pally has to put kid asleep so it is ruled he hallucinates in Cleric's place. In any case, high rolling dragonborn sees something "Glinty/shiny" in the distance so we all (save hallucinating human pally) trek towards the shiny.

Dragonborn paladin gets to the area first, and is immediately attacked by something that she(Female character, male player) can't quite see properly. We all do nature checks and a couple of us realize it is a Displacement Beast and get some basic info about it. I (warlock) get to go first, and manage to hit it doing ~18 damage. However, the cleric misses causing us to from now on only have a 50% chance to even roll for a hit [until next hit in any case]

The hallucinating paladin comes back about 2 rounds into the battle and can't see the beast, but gets a good view when it attacks him!

Besides us missing almost constantly due to the 50% hit chance ability and bad luck with hit dice, we manage to kill the displacement beast. I even had to use my shift 10 ability to get out of its threatening reach! The main thing allowing us to kill it was its three-attack ability combined with the paladins' marks causing him to take damage whenever he attacks anyone besides them (Divine Challenge). In fact, their last burst from Divine challenge causes it to explode leaving me covered in its guts, and giving me its ability to only be hit 50% of the time until next extended rest. [YAY!]

We take a short rest and spend healing surges as needed, and spot nearby a level 4 black dragon with acid breath, about 250hp [we didn't know that much detail at the time, of course]. For comparison, our tankiest paladin had 65 HP.

We charge at it screaming "We will have your head" or some similar nonsense. The dragon goes to breathe acid at us... and the DM rolls a 1, which the cleric sees due to the lack of DM screen [He promptly points to it and says "YOU ROLLED A ONE!!!!"]. The dragon manages to breathe acid in such a way to cause himself to be buried in an avalanche of snow, ice, rock, and acid. While his head was still mostly free, he was completely immobilized.

Now that we have automatic critical hits, we decide to go all out. We each use our extra action points and most damaging abilities, bringing the dragon down to bloodied in only 1 round. At this point the dragon is crying (it is a young dragon after all). Heartlessly, we continue beating it down. With ~50 hitpoints left it becomes the dragon's turn. First, he attempts to attack us again... and rolls a 2 in the attempt, causing it to have no effect. He then attempts to escape from the snow/ice/rock/acid/tears... and rolls a 3, causing nothing useful to happen. Dragon begins to cry again, harder this time. On our turn, human paladin crits on his ability and cuts off the dragon's head! We are victorious!

Our adventurers decide that in the story we tell in the taverns, the dragon will be flying around nearly killing us with its acid, and it will DEFINITELY not be crying. Also, will be a full adult.

Overall, it might be one of those stories where you had to be there but it was one of the funniest combat encounters I have yet been part of.