First let’s talk about the basic setup. The skirmish consists of three phases that players need to beat for Bronze, Silver and Gold rewards. Between these phases, the team can vote to stay for the next tier of rewards or leave the map with the current one. Each additional phase comes with increased mob difficulty and length. Bronze has level 71 encounters, Silver level 72 and Gold level 73. Bronze can be done in 3-4 minutes by competent teams, a full Gold run usually takes 10-15 minutes. The maximum possible reward for Gold is 150 Abandoned Treasures, 50 Seal of the Elements, Fleece, one random piece of Companion Equipment and some XP and rough Astral Diamonds.
The possible quality of the Companion Equipment is also affected by the reward tier. Bronze only seems to include +1 and +2 versions, Silver assumingly goes up to +4 and only Gold can net +5. All completions always net the Fleece and daily Astra Diamonds, if eligible.
When players die, they are not out of the fight. They have to walk through a Portobello-like course and earn their way back into the arena. It’s really easy once you get it. The perfect time to go by the swinging halberds is when they’ve just passed the middle point.
Teams only lose the challenge if nobody is in the arena any longer. In this case you are forced to leave without rewards. Underpowered teams therefore should carefully judge whether they can make it through the next phase.
In each phase teams are placed on a platform on a random map. Except for the visuals this however has no effect whatsoever. The same is true for the varying mob types and boss monsters. Some groups might have trouble with the scorpion you know from the Lair of Lostmauth though. Its attacks can indeed hurt. Lukan himself, who needs to be killed for a Gold completion, doesn’t present much of a challenge either.
Some mechanics however are worth talking about. In addition to the randomized map and mobs, up to three random mechanics are thrown at groups. They show up in the quest log and you always should have an eye on the active ones. Most of them are negligible or just annoying, like the control ones that morph players into chicken or turn off powers. Some however can have an impact and should be avoided.
The icy floor generally counts in the annoying category and is a nightmare for melees. The fact that you can hardly move can make it difficult to dodge certain other random environmental AoEs. So this can actually turn out quite dangerous.
“Feeling light on your feet” might be the coolest mechanic overall. It reduces the gravity in the arena and let’s players jump really high. Besides this being fun, it helps melees to move on icy floors and you also can’t be hit by mobs and AoEs while in the air. Ranged toons additionally can still attack from above.
One of the harder-hitting environmental AoEs are the poison spheres you know from the Orcus fight and burning areas on the ground. Both should be avoided by lower item levels, but do not exactly present true danger for endgame geared toons. On the contrary the falling rocks and flame thrower do comparably small damage.
The reflect damage is pretty much the only one that can constantly challenge teams, and most of the time it’s just an instant wipe. Some mobs do randomly reflect 100% of the damage and since players do dish out more than their health pool can handle, it leads to one-shots all over the place. Between a lot of really insignificant mechanics, this one seems to be completely overtuned. You should always warn your team in chat when reflect damage pops up and try to be more decisive with your powers.
The voting mechanic is a vast improvement to the Underdark tiered successes. In Demogorgon, Prophecy of Madness and such, a Bronze run would take as long as a Gold run, which was pretty annoying. This has changed now and teams have to decide in advance whether they want a higher challenge for more rewards. In most cases you should be powerful enough to beat Gold, but it makes sense to monitor your team and judge on the fly. If you barely made it through Silver, it might be best to exit.
Since it’s a majority vote, three players unfortunately can force their will onto two others. There are some players farming only Bronze for the Fleece and their daily skirmishes, others want Gold for the best shot at Companion gear. Just be aware that in pugs you’re not guaranteed to land in a run that meets your own expectation. At the very least you should communicate upfront what reward tier your group is aiming for. Or just go in with a premade.
Which brings us to group compilation. You don’t necessarily need a classic healer and tank buildup, but it can’t hurt either since those are competent buffers as well. Generally ranged DPS perform a little better because the mobs start on the outside of the arena and are mostly dead before they can reach the group. Also melees have trouble with the icy floor as mentioned.
But there are really no mandatory requirements here. Just go in and have a good time with whatever!