I think one of the highest priorities of an offensive rain team should be to eliminate Ferrothorn. Ferro walls many common rain team members including non-specs Kyogre, choiced Palkia, Giritina and Kingdra. It is particularly difficult to bring down the spike-y shrub due to his access to leech seed + protect. When I am building a rain team, I like to have a lure for Ferrothorn. Obviously the lure should have utility outside of this role and have synergy with the rest of your team.
Palkia can perform this role himself by bluffing a choice item and tagging Ferro with Fire Blast, but if you don't hit him on the switch in you are at the mercy off getting para'd or seeded, as Fire blast isn't going to OHKO Ferro in the rain. This is also not a sure-fire way to kill Ferro becasue Scarf Palkia is becoming less and less popular and people are wary of non-scarfed Palkias.
Rayquaza deserves a mention here as he can git rid of the rain and roast Ferro with Fire Blast or V-Create. While its not exactly a lure, a mixed set has a whole lot of utility as it can help dismantle many defensive cores and also sweep with Dragon Dragon dance or even Tailwind. Rayquaza's Air Lock is also a great defence against enemy swift swimmers and a safety net if you lose a weather war.
Zekrom is an amazing and often overlooked rain teammate. Both the mixed set and the Sub-Claws set effectively lure in Ferrothorn and put the opponent in a bad situation. The Mixed set is my favorite because it lures in both Ferrothorn and Groudon. Killing a Groudon early game is HUGE because most sun team will succumb to a onslaught of rain-boosted water attacks.
Rayquaza and Zekrom are a perfect example of the principle that a Pokemon does not have to directly benefit from rain to do well in it.
If you embrace the hax, you can control it. -PokeaimMD