Overall impressions:
1) It took a good 30-45 minutes to get the players to add 3 mythic tiers to their level 6 character. They really struggled to find feats they wanted to take.
2) A mythic character sheet would be a great idea, even if its just for the playtest.
3) Most of the combats were over almost immediately. The agile dire wolf got to show off his dual-initiative, but very ineffectually due to his low attack bonus. The other wolves and snakes do nothing.
4) The Treant hid the two mythic grizzly bears nicely, but once the bears go into range to attack, they weren't too effective - again, low attack rolls. The party focused down the treant first, then turned to the bears. They did some decent damage, especially some bleed on the summoner, but they died quickly.
5) The canyon flummoxed the party for awhile. Eventually, the eidolon flew back and forth, carrying one member. That stopped when a lighting bolt hit bot of them.
Next, two of the party tried to climb down the crevice and fell, taking 6d6 damage. The last one not crossing cut the rope bridge and rode it down to the other side of the canyon. Lightning struck him during this, and he failed his acrobatics.
The party then just used the now rope ladder to climb up the crevice and move on, but not before the two who had avoided lightning got blasted.
6) The hydra came out to "oohs" and "aahs." Summoner rolled knowledge on them, and got that they heal quickly, that cutting the heads off is a key to killing it, but be careful, as heads can grow back.
Sorcerer pulled out his favorite trick (same thing to a burnt out shell of a treant trunk back a few miles) Fireball at the hydra. It failed its save, and caught on fire.
The hydra didn't like what happened and charged the sorcerer. Full attack brought him to half hp with a fair amount of bleed.
Fighter tore into the hydra, followed by the eidolon whiffing multiple times.
Hydra got to act again and kept beating on the one that set him on fire. Full round attack again, and this time the sorcerer drops with 8 bleed still going.
Fighter attacks like hell and tears the hydra open, killing it easily.
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That was the end. I think most of the mythic creatures in this were a little low for the attack bonuses and their AC.
Overall, though, it was quite a bit of fun. We all agree that Amazing Initiative is absurdly overpowered at this point.