Encounter Level 9
3 six headed hydras and a level 5 wizard. (each of the 4 monsters is cr 5)
The party is attacking a major stronghold of an enemy mage. They bust down the door and stumble onto the room and find 3 hydras. They do not find the mage for he has cast invisibility before hand.
So the party tries to attack the hydras, and the hydras try to attack the party. All goes according to plan, until one of the party spellcasters tries to cast a spell. This is when the enemy mage attacks.
Instead of trying to attack the party directly the enemy mage is smart and uses battlefield control. He stays invisible until he is ready, he does not even attack when it is his turn (for if he does it too early the mages may do an immediate action to avoid his effect), instead he readies an action to cast the almighty dimension step.
When the party mage tries to cast a spell, the dimension step activates teleporting the hydras right next to the mage before he completes the spell.
Oh did I mention that the hydras have custom feats due to magical experimentation. They have Combat Reflexes (bonus), Mage Slayer, Blind Fight, Pierce Magical Concealment, and Pierce Magical Protection (in that order). Note that this is two feats too many, well in my mind what is good for the goose is good for the gaffer if your pcs play with flaws than monsters will also play with flaws. (houserule I know)
If the spell is not a swift action spell, than the mage promptly dies due to him now provoking an attack of opportunity and thus is attacked by at least 1 hydra six times. He doesn't get to cast defensively due to mage slayer, and he loses any miss chance buffs (he does not lose ac buffs for the hydra has to attack and not do an AoO for that).
If the spell is a swift action spell (and the enemy mage decided to unleash his readied action on a swift action spell) than the party mage has very limited options for if he does anything traditional then he generates an AoO and thus is dead. He can 5ft step but he is still in the range of the hydra (15ft range). He can try withdrawling but since the hydra has a range of 15ft his first square may be safe from an AoO but his second and third square are not and thus he is dead if he tries withdrawling. His only option is doing a swift action spell and hope that saves his butt.
After casting said spell the mage is still invisible since he didn't attack directly, so he can repeat this tactic for the next round, perform another spell, or run away.