This happened during my first year of collage around 2012 and incidentally was my and my two brothers introduction to D&D and RPGs as a whole. Due to and busy schedules it was very difficult to run a game then but we found time between and after classes in the very large commons room or the side room used as game room. Though we never really finish a campaign if was still fun and interesting to a then newbie to the gaming community.
At the time I was playing with my two brothers and a small group of experienced or semi-experienced players. To begin I don't remember everyone's classes but in can infer looking back what they must have been due to what they used. I was a Lightning Breath Gold Dragonborn Wizard of Necromancy, one of my brothers was a nondenominational Dwarven Cleric of the Life and Death domains, my other brother was a High Elf Ranger, we also had a Human Rouge Thief and a Gnome Fighter.
The Colosseum in question was a side mini game the DM ran outside our main sessions to add to the power and loot players had in game. It worked like a video game were participants would fight mobs of predetermined enemies as a group and those that survived would get points more than those that didn't and we could use them to buy items or enchant ones we already had.
To give a bit of a visual the arena of the Colosseum was round, about an 80 ft radius with 20ft tall exterior walls, and had a large rectangular pit roughly 25 feet deep and about 20 ft x 30 ft with four stone pillars at each corner.
In this instance the party in question was to fight a mob of Orcs, a few Ogres and a Young Red Dragon that was chained to the ground with about 200 ft of chain to still be free enough to move and fly. The party upon realizing this was quite surprised due to only fight hordes of Undead and such before so it seemed the DM was giving a much more intense challenge. At it was then that the party soon fell into chaos.
At the start the Gnome Fighter showed that not only in game but out of game he was a bit of a coward as he ran to the pit and climbed in to escape the dragon just before the Rouge and Ranger were blasted with fire by the now flying Red Dragon. They in turn retaliated by the Ranger firing volleys of arrows at the Orcs before the Rouge began to climb the chain to get to the Dragon's back and attack.
By now I saw that this whole fight was going badly so I started on my hail Mary a ritual casting of Summon Monster. The Cleric decided to help me and rolled high on an arcana check so he cut the one minute cast time in half. By the time the casting was done a good majority of the Orcs and Ogres were dead and the Fighter had still refused to engage anyone due to the Dragon. Unfortunately the summoned creatures were less than ideal, most likely because the DM was screwing with me. As it turned out I had ended up summoning a Lich and a Balor both of which unsurprisingly refused to obey me.
So in my desperation I had only added to our troubles. Though the Cleric soon came to by add and with an intimidation check, and an added bonus for a connected punch, convinced the Lich to follow my orders. Upon my next turn I ordered the Lich to attack the Dragon before moving to the Balor in hopes of making a deal to help us or at least leave. Though before the demon could even finish uttering the words "Why don't we make a deal?" the Cleric rushed and punched it trying to intimidate it like he did the Lich. But dispite his good rolls the Balor refused to bend the knee and it was left up to me to try all the which fighting continued around us and the Ranger was running low on ammo. So even more desperate to get a hand on the situation I took a page from the Cleric and tried to intimidate the Balor by backhanding it while casting Vampiric Touch with the highest spell slot I had, fifth level I think. I succeeded with a crit and did almost max damage, healing me fo much needed health and making the Balor finally submit it my will. Now it was time to take on the Red Dragon with a fervor.
At this point the Lich had fallen into the pit due to a shove for one of the last Ogres, the Rouge was still on the Red Dragon, all the Orcs were dead, I was about out of spells same as the Cleric, the Ranger had maybe five arrows left and the Fighter was still in the pit. I and the Cleric then used the last of our spells to take out the last Ogres leaving only the Dragon left alive.
It was then the idea to make the dragon fall into the pit it was over hit us, or rather the Ranger got the idea. He, the Cleric and I worked to cripple the wings while the Rouge continued to slash at the Dragon from it's back nearly falling off a few times. During this time, since my spells continued to miss, I had tried and barely failed to hit the dragon with my lance which ended up on the opposite side to the arena. The Cleric then stepped in and threw two javelins at the Red Dragon's wings finally doing enough damage to the wing to have it fall.
As the great beast fell the Rouge managed to jump from it's back and land at the edge of the pit while the Gnome fighter took massive amounts of damage. The Balor and Ranger being next to the pit were less lucky as the two fell into the pit with the dragon and were crushed under it along with the Lich which was promptly killed. The Rouge then jumped on top of the Dragon again in the pit to continue his assault while the Ranger out of ammo used his short sword and the Fighter miraculously began to do something useful and fought the Dragon.
Now out of options and spells the Cleric and I did the only thing we could think of; push one of the pillars over and into the pit. The first turn trying to do so failed but the second bore fruit at the large pillar fell crushing part of the Dragon and decimating the Balor and the Fighter leaving only a puddle where they once stood. Running around to the other side to a pillar closer to the Red Dragon we pulled off the same trick to finally slay our foe and unfortunately the Ranger with it. High-fives were had for those still alive and points distributed to the living and dead, though none to the Fighter as the DM rightly ruled he did little to nothing to help anyone. So in the end we leveled up quite a bit, gained a few new enchanted weapons and survived the Colosseum yet again.