Dratini, go for a Thunder wave on Spheal, then follow up with a light screen. Then, use Thunder at spheal. (Even if it misses and hits the water, spheal will still be shocked!
Thunder Wave (Spheal)>Light Screen (My Party)>Thunder (Spheal)
Firion, Intimidate both of them to weaken the physical attacks! Then use Close Combat on Spheal before it rolls into the water (because he rolls away, he won't have the advantage of being close to you!) and then use Helping Hand to power up Dratini's Thunder!
Intimidate (Both)>Close Combat (Spheal)>Helping Hand (Dratini)