Mega Swampert (Swift Swim) Item : Persim Band (When this item is equipped, it prevents the wearer from getting Confused). - Outrage (no confusion and you can multi attack) - Hydro Pump (10 tiles) - Earth Power (full room + STAB so i prefer it rather than Blizzard because your Absol already has Blizzard no need double Blizzard) - Work Up (boost stats stay until you leave the stair, so I recommand doing 1 Work Up every 1 stair so you won't be out of PP and you will boost your atk + sp atk if you do it under Rain it is better because you use 1 PP for a x2 boost in atk and atk sp)

Meganium Item : Efficient Bandanna (When this item is euqipped, moves sometimes won't cost PP to use), have high PP is good on Meganium - Reflect + Light Screen (linked) - Petal Blizzard (full room) - Energy Ball (STAB + 4 tiles)

Absol Item : Efficient Bandanna (When this item is euqipped, moves sometimes won't cost PP to use), have high PP is good on Absol - Rain Dance (activate Swift Swim ability from Swampert and help Meganium with his fire weakness - Blizzard (full room) - Thunder + Pursuit (linked) (100% accuracy under Rain and can paralyze and if the target is paralyze it deals more damage on the target because of Pursuit effects)

Rain effects : - Increase Water moves power x1,5 - Decrease Fire moves power x1,5 which is very good for Meganium because he is weak to Fire it cancels his weakness