What is the lowest level AC a Player Character can have an AC of 39 (so a Tarrasque can't hit them without a critical hit) with help from at most one same-level ally?
No magic items may be used that the characters don't create themselves (like from Artificer or Forge Cleric for example), but all official class features, races, feats, multi-classing, and abilities are on the table.
The second character must be the same level as you.
You are also allowed to prepare beforehand, and you get as much time as necessary, so long as the characters won't die from old age.
If a certain roll has to be made (like for a Defensive Duelist), assume you roll the average roll every single time.
Show me a sustainable 3-5 round combat version.
Also don't include cover.