Fight one: Ten years of prep, both of them. Thanos doesn't get the infinity gauntlet or heart of the universe.
Fight two: Thanos is transported to the Tenchi universe universe with ship and everything he can carry on it. He can't have the infinity gauntlet or heart of the universe. He and Washu have fifty years of prep.
Fight three: Thanos is transported to the Tenchi universe with his ship, given a copy of washu's Tech and has one hundred years to study it, afterword they both have hundred years of prep to deal with each other