You have a pretty awesome team, no doubt about it, but there are few rather odd choices in this team, firstly as a Trick Room team, and secondly as a Rain team. As much as I like the starting two you send out at first, I do have some suggestions.
Blastoise is definitely a good pick for a Rain team, especially since it has Water Spout, but probably not the greatest pick for a Trick Room team. With it having Fake Out, it can definitely stop a lot of things from happening to your Aromatisse to set up whatever strategy you need, but, with its fairly average Speed, there's no guarantee that it's going to work well with Trick Room, but as you have somthing using Heal Pulse next to it, it's probably worth using. Greninja might be a consideration to protect your Trick Roomer as it has Mat Block, and is the fastest Water-type, so it can probably give the best initial protection, and you can give it an Eject Button to switch out afterwards instead of having something else take damage. However, as Mat Block is not Priority, it is ironically vulnerable to Fake Out, and after Greninja is used, it is definitely not for Trick Room use. So I'd stick with Blastoise. As for Aromatisse, it is definitely good to go in terms of moveset, but I'd definitely suggest Slowking over it, especially if you are going to use a Rain team. Slowking can learn everything that your Aromatisse can learn except Dazzling Gleam, and if you replace that with a Water-type move, you can use the STAB and Rain boost a lot better. Slowking also has better defensive stats (especially Special Defense), so you might want to consider that. The rest of your team looks alright, but the initial idea was a Rain team, in which Aromatisse, Aegislash and Chesnaught fit into rather strangely. The only thing the rain can be used for after Blastoise is Goodra, so you might want to add something that can use it. You also might want to add another Trick Roomer in case your Aromatisse get unsuspectingly wiped out at the start with a Steel or Poison attack (even though both are pretty uncommon).
From my viewpoint you're actually pretty good to go, you just need to add the last member of your team. You said you were looking for a bulky Water-type, so here are my suggestions:
Octillery - Since you're using Trick Room, this is my best suggestion. Octillery can learn a Special Attack from practically any type (it can even learn Flamethrower and Fire Blast), and can learn Water Spout, just like Blastoise. It also has a very high Special Atk. stat. If your Blastoise does happen to faint, this is a good backup. However, it's defensive stats are pretty laughable, so it's not necessarily going to last long, along with its Water Spout, but it's definitely one of my first Trick Room choices.
Slowbro or (as mentioned before) Slowking - Both can do the exact same thing as Aromatisse, have better Defense (bro) or Sp. Def. (king), and are immune to taunts to get Trick Room working (if you have Oblivious). Slowbro can also learn Belly Drum, but it isn't necessarily the best choice for a strategy like that. Either can be used as backup (or replacement) for your Aromatisse.
By the way, thanks for not using Politoed. I'm sick of seeing people use that for Rain teams. But even if it has Drizzle, I still wouldn't use it for Trick Rooming.
As a side note, I have a Triples Trick Room team that's somewhat similar to this. Maybe you'll get some ideas from it:
http://pokemondb.net/pokebase/rmt/29975/rate-my-monotype-triples-team
