I've DM'ed 2nd edition 15 years ago, but this is my first time with Pathfinder. To start off, I'm working with a group of 4 DM's to guide 4 groups through a combined adventure, and I'm in the arctic. I've been focusing on cold plains, cold mountains, and underground creatures for the most part. The adventure started at lvl 4 (5 PC's), they're currently at level 8, and I've been running encounters with up to CR7 with some challenge, but for the most part it hasn't been overly difficult.

The question that I have is at CR8 is there a power spike? I've yet to find a decent CR 8 monster for them to fight. I was intending on throwing a Giant Tarantula at them, but a 20 ft reach, that high of health, and that much of an attack bonus, the PC's wouldn't be able to deal with that. After that, I just browsed through CR 8's, and there is the Bodak, Axiomite, Morgh, Two-headed troll, a bunch of Dragons, and other nasty looking creatures. All of the CR 8 creatures I look at wouldn't need to roll dice to hit most of the party, have a high AC, some amount of SR, and would be able to 1 hit many members of the party with a full attack.

So am I missing something, are they under-prepared/equipped for their level, or is this just of the game designers leveling the playing field a bit?

Thanks in advance.

Edit: Posting at 2 AM, I just now realized that I forgot to mention that they are currently a level 6 party, on the verge of making level 7. I'm not throwing CR 8 encounters at my level 4 party, I'm not "that" evil.