New GM making a boss[PF]

[Stone Heart]

I don't know what level to make a boss for the end of this dungeon, its the first time I have been the gm for long enough for this to come along. The party is all 6th level and consist of a Summoner, a Rogue, a Cleric, a Monk, a Fighter, a Druid and an Alchemist.

One of my problems with this is that I don't understand how challenge rating works in relation to level, and not everyone shows up every meeting, like I know the monk won't be there, its likely the druid won't, and nobody knows with the summoner.

So if I am making a character for them to fight what level should I make the character?

[Crossblade]

Read Pathfinder Core Book, p397; Step 1 - Determine APL.
Short version:the average CR is for a party of 4-5 PCs. You have 7 normally, but it's likely you'll have 4-5 for that session. So by default, you can make a normal CR fight. The table on page 397 gives ways to determine how hard you want the fight.

Also note, normally, since you have more PCs than the standard 4-5, it says "for groups of six or more, add +1 CR"

So, if you want a Hard to Challenging fight, throw a CR 8 encounter at them.
(average level 6, for 4-5 players; plus 2 for Hard) or CR 9 if you really want to make it extra hard, or think the summoner may show.

The formula on determining how to make an encounter a certain level is on page 398. Basically, in the Beastiary, all monsters are given XP. Just add up their XP until it's equial to cr 8 or 9 (4800 or 6400 respectfully).