Unless your a Gym Leader; never use mono-typing. You'll die.
Poli is great but if your placing EVs in Def. and SDef., don't bother with SAtk, just spread on HP. Plus 4 doesnt go into 170, so you're waisting perfectly good EVs switch your defenses to 168 each and place the rest on HP. Shell Bell won't help... unless you've got that extreme Politoed on your hand. Lefties is the way to go. Switch Modest to Bold. Hypnosis to Toxic. H.P. Elec. to Scald.
Ludicolo looks like roadkill. Split the EVs the exact same way as Poli. Timid to Bold. Scald over Toxic/Surf. Protect over Drain Punch and Rain Dance.
Your last Water Pokemon will be Gorebyss, a wonderful Shell Smasher. With Swift Swim banned your forced to use RestHydrate. Ice Beam and Scald are the best, IMO, for his attacks. 252 Atk and Spd, 4 HP. Modest. Focus Sash.
Now for the Others...
Dragonite: He will be the one to just completley abuse the rain. You have 2 options: Choice Specs/ Hurricane/ Surf/Thunder/? (Come up with a new attack with a different type). Or Lefties/ Surf/ Hurricane/ Thunder/ Roost. No matter what, you'll be using Multiscale and your spread will be 204 HP, 96 Def, 152 SAtk, and 56 SDef. Modest nature is the best nature for him.
This Magnezone is very original. Use Metal Sound to destroy their defense. If they switch, we've got a plan for that. Thunder refuses to miss in rain and HP Water gets a power boost. Lastly use some protection with either Protect or Substitute. Give him an Air Ballon to avoid his terrible weakness with ease. HIs EVs are elaborate: 80 HP, 136 on his Defenses, 156 on SAtk. Quiet Nature. An Analytic Power-Boost is great.
Jirachi will be your lead. This will be a rare set so not many can check it. Doom desire. Iron Head. Thunder Wave. Wish. 252 HP, Split the defenses. Kings Rock. Sassy.