The maximum damage has remained unchanged at 65535 for a long time, and no amount of playing around with Forest's Curse or Tar Shot will let you go beyond that.
For a simple way to hit that absolute maximum on turn 1, you could have a level 1 Tirtouga with minimum SpD (4 points) lead off the battle with a Quick Claw Shell Smash, reducing that stat to 2. Then its opponent, a level 100 Sniper Octillery with Scope Lens and 335 in the Special Attack column (note this is NOT the maximum stat it can have, which is 339) uses Water Spout and gets a critical hit. The damage rolls for that are all over the place, but one of the figures right in the middle hits the unbreakable summit.
Yes, Tirtouga is neutral to Water. Yes, there are possible Shell Smash users like Shuckle or Torkoal who would be weak to water. No, switching the defender to one of those would not be an improvement, nor would giving Octillery a boost item like Choice Specs. Such are the things you have to take into consideration when overflow avoidance shows up on the scene.