General rules of the Crucible
The [Crucible] is a special location which isn't connected to any main or side quest. It's an arena where you can test your skills by battling groups of monsters and winning valuable rewards. You can travel to the Crucible regardless of the currently occupied area, because it's visible on all four world maps.
The Crucible isn't available from the very beginning. You'll be able to travel there only after defeating the Guardian, the final boss of [The Heart of the Mountain] main quest. As soon as you take him down, you should receive information about a gift from an anonymous sender (the above screen). Go to any Serpent's Tome (i.e. the one on Tri-Stone in the Forge Lands) to read the message and receive the first Tarot Card (The Fool).
Upon arriving at the Crucible, listen carefully to the fight manager as he explains the rules of this place. After five won battles (waves), you'll be asked whether you want to continue fighting or stop and receive your current rewards. The first answer seems more logical, BUT if you die, you'll lose the chances of receiving any rewards whatsoever. The only exceptions are "permanent rewards" which are awarded after every 25 waves, regardless of whether you decide to continue or not.
Keep in mind that completing all battles IS NOT possible when you're playing the game for the first time. Here are the details on how to unlock the subsequent waves in the Crucible:
- stage 1 (waves 1-25) - collect the Tarot Card after defeating the Guardian ([The Heart of the Mountain] main quest)
- stage 2 (waves 26-50) - collect the Tarot Card after defeating the Tormentor ([The City of the Dead] main quest)
- stage 3 (waves 51-75) - collect the Tarot Card after defeating Samael ([The Lord of the Black Stone] main quest)
- stage 4 (waves 76-100) - collect the Tarot Card after reaching level 25 - you can advance that far only in New Game + (after defeating the Avatar of Chaos and ending the main campaign); Death can only reach level 22 in the first playthrough
- stage 5 (additional boss battle with Wicked K.) - complete all 100 waves (stages 1-4) at one go
How to survive?
As mentioned earlier, completing the Crucible isn't possible during the first playthrough. I suggest going back to it only after beginning New Game +. Nevertheless, I don't really recommend rushing to the Crucible as soon as you reach level 25 (required to unlock the fourth stage). Take your time to build Death to level 30 and acquire the best gear possible. I don't suppose I have to repeat that you should also use properly upgraded Possessed Weapons. Apart from that, try to amass items that raise your strength and critical damage.
As for the skills, I would mostly advise to focus on the Harbinger Tree. My favorites are Teleport Slash, Unstoppable and Harvest, with all associated passive skills. You should buy skills from the Necromancer Tree only if you have some loose Skill Points available. If so, I would recommend the active skill Exhume, with all associated passive skills. Note - If you decide that you've built Death the wrong way, you can pay a visit to the merchant Vulgrim and buy Respec (the above screen) - this will allow you to redistribute all of your Skill Points.
The battles in the Crucible start pretty innocently, allowing you to break through the first couple encounters without much trouble. In time, though, more powerful monsters will appear on the arena, and in larger numbers. At the start of each battle, I suggest instantly eliminating the weakest opponents, so that they don't bother you during the rest of the struggle. When they're down, focus on the stronger beasts, using all of the tricks learned throughout the main campaign and during side quests. Try not to drink Health and Wrath Potions too early, but rely on regenerative properties of your gear instead. It's also a good idea to use the Reaper Form as often as you can, especially given that during the more complex battles it can charge up again pretty quickly.