God that was a slog, but it's my first 200/200 game so I'm well happy. 13 is indeed an absolute ***, 14 starts the 4-team games and the difficulty drops down for a while before gradually building back up to 20. I think when I beat 13 I just moved all my worms to one side of the map with a tall landmass and wiped out any enemy worms on it. Then I burrowed, making sure to twist the tunnels cause the AI are ridiculously accurate. If a shot is physically possible, they will pull it off. So make it impossible, tweak the left analogue up and down while blowtorching to put bends in your tunnel that will stop grenades and rockets.
Burrow deep but make sure you can get back out cause you're going to want to head toward the surface when the water rises. Unfortunately 13 has a roof, but 20 does not, so it helps to save an airstrike till the very end of sudden death to eliminate anyone who has higher ground than you. Use girders to shield yourself yes, but also to box in any troublesome enemy worms, or if possible a group. Unless they teleport out or blow themselves up, it'll usually keep them in place till the time runs out, at which point you can poke your head out and blow their *** up with a homing missle or Zooka shot.
On 13 and 20 cowardice really is the best policy, if you burrow deep enough and smart enough you can just walk away from the console until the time ticks down. I've seen the AI make limited attempts to find higher ground, but generally they'll drown long before you do.
