Hi,

I'm currently creating my first big campaign and have designed my own BBEG. I've searched online to find some answers for what an appropriate CR conversion is for PC's, but i've not found anything good. Maybe you guys could help me. 

Link to character sheet here: Mirthun.

I would like to know what level my party of 6 has to be to make this a 70-30 winnable fight.

I would recommend using a statblock, not a character sheet. PCs are extremely squishy, so your boss will likely die in one or two rounds. Using a modified archmage might be a better idea. With that said, I think that your players would need to around level 9 to win the fight with those odds.

When I bounce Mirthun through DMG - Ch. 9 with a couple of assumptions about how they will be used, I come up with a CR 10. YMMV, feel free to run them throug as you would run them.

Deadly encounter threshold for 4-Lvl 6 PCs would hover around 5,600 XP, given that you use your daily XP budget of 16,000 XP. Which, as you can see, is about 1/3rd of the day. Will probably be a cake walk. A single CR 12 Archmage will probably be winnable, but on the cusp of deadly. Initiative will most likely be the deciding factor. If you were to throw some minions at the party first to complicate them and "soften" them up, this will most likely be a deadly (read as: possibility of PC death, not a certainty of PC death) fight. 

I agree with Thauraeln that a stat block will be easier to manage for you as DM. It wouldn't take much to convert your Char Sheet to an NPC statblock.

Here's the issue you'll have:

With the kind of firepower that a level 18 wizard can put out, you effectively have a 'hard floor' as to how low level the PCs can be. The hard floor is whether they will survive a single turn.

Let's assume that Mirthun is going to open any combat with the PCs by casting Sunburst hitting them all. That's 42 (12d6) radiant damage on a failed save, or 21 on a success. Assuming that they won't all save, any PC with 42 hit points or fewer on the first turn of combat is going down. Assume a +2 Con modifier, for our squishier classes (wizards, sorcerers), our squishies need to be at least 8th level to reliably survive one blast. However, on the next turn he'll follow up with circle of death for a further 31 points of damage on everyone, so by turn 2, everyone needs to be at least 11th level.

However, here's where we get some real issues with running a PC as an NPC. counterspell ruins his plans, making his whole turn irrelevant if the PC rolls well. Not to mention that he won't likely keep up any concentration spells with a barrage of attacks inbound.

By the time the PCs are able to survive his AoE damage, they'll also be powerful enough to nuke him in 1-2 turns. This is why you want to make a monster stat block rather than use a PC character sheet.

Personally the boss monster that I'm using against my party of level 13 PCs has over 700 hit points so that it will be a really epic fight.

Quote from Sanvael >>

Here's the issue you'll have:

With the kind of firepower that a level 18 wizard can put out, you effectively have a 'hard floor' as to how low level the PCs can be. The hard floor is whether they will survive a single turn.

Let's assume that Mirthun is going to open any combat with the PCs by casting Sunburst hitting them all. That's 42 (12d6) radiant damage on a failed save, or 21 on a success. Assuming that they won't all save, any PC with 42 hit points or fewer on the first turn of combat is going down. Assume a +2 Con modifier, for our squishier classes (wizards, sorcerers), our squishies need to be at least 8th level to reliably survive one blast. However, on the next turn he'll follow up with circle of death for a further 31 points of damage on everyone, so by turn 2, everyone needs to be at least 11th level.

However, here's where we get some real issues with running a PC as an NPC. counterspell ruins his plans, making his whole turn irrelevant if the PC rolls well. Not to mention that he won't likely keep up any concentration spells with a barrage of attacks inbound.

By the time the PCs are able to survive his AoE damage, they'll also be powerful enough to nuke him in 1-2 turns. This is why you want to make a monster stat block rather than use a PC character sheet.

Personally the boss monster that I'm using against my party of level 13 PCs has over 700 hit points so that it will be a really epic fight. 

 To add on to this, I think that the best bosses have high health and defenses, but only moderately powerful attacks. That way, they'll take a long time to go down, but it also won't be super likely to be a TPK.

There's a realistic chance of taking this guy out with around a level 5 party, it just depends on initiative. If you have, say, an open hand monk who wins initiative, he's got a pretty good chance of landing a stunning strike, at which point a reasonably cautious party can make sure he never gets an action. However, if he does get an action, great, he casts Prismatic Wall and starts playing popup games with Invisibility and the party is likely hosed. It's a general problem with high level wizard enemies, they're glass cannons, which don't make terribly good boss fights.

To add some further advice, if you want to reduce the chances of a TPK, you can do things like split up the damage. Not only does this prevent one party member from being nuked down, but it also makes the danger feel real for everyone as everyone is being attacked in turn. You can flavour it by saying things like. "Alright, well Mirthun really wasn't happy by being attacked by the wizard so he's going to attack," and have him switch targets etc.