Okay /vp/ I need some help. I have a 4 pokemon core that I'm in love with but need some help devising the last 2 members.

Rotom-H @Chesto Berry
Bold, 252 HP, 252 Def, 4 Sp. A
~Volt Switch
~Overheat
~Will-o-Wisp
~Rest

Venusaur @Venusaurite
Calm, 252 HP, 4 Sp. A, 252 Sp. Def
~Giga Drain
~Sludge Bomb
~Leech Seed
~Synthesis

Gastrodon @Leftovers w/ Storm Drain
Relaxed, 252 HP, 252 Def, 4 Sp. Def
~Scald
~Earthquake
~Toxic
~Recover

Magnezone @ Choice Scarf w/ Magnet Pull
Modest, 252 HP, 4 Def, 252 Sp. A
~Thunderbolt
~Hidden Power Fire
~Flash Cannon
~Volt Switch

The goal of the core is to have no one share weaknesses and be able to take out any threats that come their way. Gastrodon, MegaVenusaur, and Rotom-H all resist each other's weaknesses and Magnezone is to trap the steel types that would wall Venusaur. Any ideas for the last two slots? I'm not opposed to giving Venusaur Black Sludge in exchange for another Mega, but the added bulk is almost too good to pass up.