For DeXter:
I am going to suggest using this Gastrodon.

Gastrodon @Leftovers
Storm Drain | Sassy
252HP / 252 SpD
-Scald
-Earth Power
-Ice Beam
-HP Grass / Recover

This guy is a great check for any special attacker not using a Grass move. He synergizes well with any teammate that uses Surf, as he gets a +1 Special Attack boost every time he gets hit with a Water-type attack. He can survive a Draco Meteor from Lati@s and OHKO with Ice Beam. HP Grass is recommended to surprise bulky Water-types, but Recover can be used instead if you're concerned about longevity. If you want a more instantaneous Sp. Atk boost, you can use Choice Specs, but this is not recommended. Gastrodon is very physically frail, so you should use him as a late-game sweeper or as a lure for opposing physical sweepers.

An alternative set for Gastrodon is (albeit I'm not sure if Gastrodon at all will really fit in this team)

Gastrodon@Sitrus Berry
calm nature (+ Sp.Def| -Att)
split: 84 HP | 92 Def | 80 Sp.Att | 252 Sp.Def
ability: Stormdrain

• Scald
• Icebeam
• Earth Power
• Recover

The moveset is pretty standard, but benefits of the spread are notable, since it does not only survive gem-meteors from Latios and Hydreigon, but also adds in some physical bulk to better tank resisted or neutral hits like Terra's gem Stoneedge and Tars Crunch (it takes <50% from both) A bit of Sp.Att investment ensures a few OHKOs (Earthpower vs. -1 Terra f.e.) even without Stormdrain boost, Latios and Tar are both possible 2HKOs without boost (and sure 2HKOs with it). The Sitrus Berry allows Gastrodon to survive the Gem meteor + a possible consecutive -2 meteors, which makes it virtually impossible for Latios/Hydra to 2HKO this Gastrodon (at least without hax).

To the team itself, I have to admit, I'm not very pro at doubles, but IMO is looks very solid. Maybe the spreads are a bit too generic for my taste, but the concept and the type coverage look really good.
Perhaps helping hand > focus miss on 'toed might be an idea, to avoid bad accuracy hax and have an option to provide a bit of teamsupport. Especially since your toed is meant to team up with toxicroak you already have the option to deal with fighting weaks, and hh might help croak to score KOs more easily (and recover more hp as well).

I actually tried gastrodon on this team in the trial stages. Its very difficult to place on this team. Who do u suggest i switch out or partner it up with?

Due to the fact that it has Storm Drain, it fits well with any water-type on your team. Also, just as another personal preference, I like using Earthquake instead of Earth Power. Not only does it make Gastrodon a bit more versatile, but it also synchronizes well with Rotom and Dragonite.

Gastrodon-252 HP/4 SpA/252 Sp Def
Leftovers
Storm Drain
Sassy

Earthquake
Ice Beam/Hidden Power Grass
Recover
Muddy Water/Scald

As stated above, really difficult to say. As much as I like Gastrodon (which is alot :D), I don't think it would be a clearly superior choice over anything in your team. Maybe Ferrothorn, since it is IMO the least useful member concerning your offensive synergy, but its steel typing offers a lot of useful resists. Gastrodon might be a try over it, since it handles most rain threats about as well as Ferro and can definitely stand special based dragon moves.
As chortler already said, offensive synergy with rotom, draggy and politoed is pretty good.

Yeah i was thinking about that. I would replace mamo for gastrodon and give him earthquake. Then i would take out hydro pump from rotom and give it surf instead.

If you decide to replace Mamo with Gastrodon, you will need to change your strategy of dealing with Dragon-types. It's definitely doable, but you'll need to more carefully preserve Ferro (which shouldn't be a problem) as you won't be able to priority OHKO dragons, and as a result, will need to tank more Draco Meteors and Outrages.

Hmmm true. I forgot about those dragons...

I can't imagine that they could be a huge problem since you would have Politoad, Dragonite, and Gastrodon equipped with attacks that can effectively deal with them while also being bulky enough to take a hit or two.

It might be worth putting a few EVs in Spe to outspeed other uninvested, neutral Dragonite.