How do you determine the challenge rating of an encounter in 4e?

Question

Recently my players decided to make the game as hard as possible for themselves by planning a jailbreak that will make them fugitives before they complete their mission to spy on the King. To make matters even harder the minotaurs in the world are supposed to be jailed and killed on sight by order of the King... and it just so happens the person the group is saving is a player character that happens to be a minotaur. So, the group of fugitives with a minotaur is going to have to stealth around a highly populated city and the surrounding areas spying on the King of all people.

When the players decided that was the best idea I told them that I was going to make the game as hard as I possibly could (while still giving them a chance to succeed). Now I am faced with a level 5 party of a Minotaur Shaman, Dwarf Cleric, Human Wizard, Human Sorcerer and possibly a fifth member depending on his schedule.

So this leaves me with about 4-5 players at level 5. What is the hardest encounter I can throw at them that still gives them a chance to survive? is a level 8 solo creature to strong for 4 level 5s? is there anything in the DMGs I am blind and am missing regarding this?

NOTE: Everyone is agreed that the challenges are likely to be deadly, so I am looking for a way to supply enemies in my encounters that have a good chance of killing the players and will be hard to take down.

Answer 1

Check out this related question Is a high level Elite the same difficulty as a low-level Solo, as XP suggests?

use the xp encounter guidelines laid down in DMG2 to craft an appropriate encounter and only use enemies within 1 lvl of the party (4-6 in your case)

The key reason enemies should only be within a lvl of the party is that 4e's very planned progression/math expects certain numbers for to-hit, defenses, health and damage for both the PCs and monsters. If either side is too high compared to the other they will avoid being hit and land lots of hits dealing a lot of damage to the smaller health pool of the lower level side.

As you can see for 5 lvl 5 PCs an encounter budget of 1000xp is considered standard. A lvl 5 solo would perfectly match that xp budget.

Once you've found the standard XP for your party size and lvl then look at the difficulty table to increase or decrease the xp budget accordingly

Scale it up, make the budget 1250 or 1500 and use a lvl 5 solo mixed in some other enemies to really put on the pressure.

Use the standard budget, but pick enemies that prey on the party's weaknesses

The party might be too ranged-centric, rely on too many power combos or have a specific NAD that is quite low. You can make things more difficult by creating encounters specifically to undermine their strengths and exploit their weaknesses.

Answer 2

I can only suggest going to an encounter builder, such as: Kassoon's 4e Encounter Builder (which, by the way, says that a level 8 solo's difficulty is "extreme" for a party you've described).

Otherwise you'll just have to solely rely on knowing your player's play-styles, game rules, and best practices for making encounters without the guiding hand of an encounter builder. As always, your judgement is key to making successful encounters, but encounter builders help. (Warning! Subjective Input Follows!) This is especially true in 4e, where the level of monsters seem to be fine-tuned to provide certain challenges for balanced parties, although exceptions still occur.