Someone may have pointed this out but grappling does not stop spell casting. It forces you to make a DC 20 Concentration check, which for most casters is pretty easy at the levels you are talking about. Sure, you need the component in hand and it can't have Somatic, so it's difficult without either Still Spell or Eschew Materials. There are still some great spells which are Verbal only, soem have already been mentioned (Personally IMC I allow Somatics with the concentration check when "Grappled" but not while Pinned...)
You say a "high level" Evoker / Eldritch Knight caster could do, but don't specify a level.
Blink - You can make your moves while not on the material plane and exit the tentacle's area.
Tenser's Transformation - Bump Strength and gives a Fighter BAB - you aren't casting spells though with this, so it is ideal if your Evoker / Eldritch knight has already cast all his "buff" spells and is ready to throw down.
Is Contingency an option? I don't think making a Contingency based on being immobilized or grappled would be too out of place for any high level mage given the pain it can be.
Take a level of Cleric and get Travel Domain : )
Ask the caster to turn off the Tentacles? Nicely? (Suggestion - Verbal only)
How high a level? Disjunction is V only : ) Prismatic Sphere...Wish...Open the fight with Mislead / Flight, etc. etc. I'm probably getting outside the level range here, heh.