Problem: Toxicrak is Poison/Dark... isn't it?
The team is still great though even without the toxicroak, just get the quilfish to use poison spikes and toxic to really hurt any non-steel types and surf, hydro pump and thunder to mop up the rest. Kingdra is also a good choice due to it only having one weakness and the Vaporeon is a very good choice (have you seen how high some of it's stats go at a high level?) and can cause quite a shock when it comes out with something like psychic or energy ball (via breeding... well, i'm sure eevee can get those from it's parents...). Lantern is also good as it has the potential to take out most of the starters (apart from the grass types).
My point with Toxicroak was that his ability(Dry Skin) lets him get 12.5% of his health back in the rain,so he kind of stays with the rain theme.
And with Leftovers he almost gets free subsitutes,only losing 6% of his health every time he uses one.
He's Poison/Fighting,not Poison/Dark.
Would be something like:
Lanturn/Damp Rock - Rain Dance / Hydro Pump(Surf) / Thunder / Thunder Wave(Ice Beam)
Kabutops/Life Orb - Stone Edge / Waterfall / Aqua Jet / Return
Toxicroak/Leftovers - Subsitute / Focus Punch / svcker Punch / Stone Edge
Vaporeon/Damp Rock - Wish / Protect / Surf / Rain Dance
Kingdra/Life Orb - Rain Dance / Draco Meteor / Outrage / Waterfall
Ludicolo/Life Orb - Surf / Energy Ball / Ice Beam / Rain Dance
Didn't include EV's because I'm lazy and they should be obvious.