Well, it's a common strategy in building in Obbies or Climb, and yes, it might seem impossible to get to the sentry, but remember: You have something called Rage, and you can also try to "cheat" in climb/obby (example is in cavernjail).
If you're Saxton Hale, try to build up rage using the sentry, and SJump on the sentry and use Rage.
If you're HHH, use the teleport ability to teleport inside the engi, and try to not fall off edges/obstacles, your priority is the engi, so if you kill the engi, you also destroy the sentry.
If you're Vangineer, Use Rage to get to the Sentry (idk if Vangineer's Rage neglects knockback, but if knockback still works when rage, then just do what I said in Saxton Hale, Build up Rage, and SJump.
If you're CBS, it's pretty easy, build up Rage, use Rage and snipe the engi with your huntsman
If you're Easter Bunny, the bombs don't work because it'll just fall off the edge or something, so just do what I said in Saxton Hale.
In all honesty, just deal with the engies before dealing the enemy team. So that there would be no hassle (dr spies intensifies)