Geyser

StrikesElec+WispFire=Electric Wisp

Wisp picks up the lightning charge - homing damage that arcs to nearby targets.

StrikesElec+BubbleAcid=More Bubble

Charged acid grows a bigger bubble field with extended duration.

RainAcid+StrikesElec=Lightning Strikes

Rain conducts the lightning - chained strikes hit everyone in the rain zone.

Cactus summons (turret, wall, mine, decoy, golem) inherit elemental damage when struck by a Fire or Elec spell. The same trick works with elemental gunshots: a Pistolero hit by an Elec-effect bullet starts firing electric rounds.

Any Fire or ElecIgniter / Charger+ThrowerAcid=Acid DPS Burst

Hit a Fire- or Elec-affected target FIRST, then Acid Thrower on top, and the acid detonates into a giant continuous damage burst. Reversing the order (Acid first, then Fire/Elec) only triggers a single tick.

Cactus summons hit by a Voodoo aura (Drain, Rescue, Corruption) turn purple and spread that corruption to every enemy they strike - the stain is transferable between summons and detonates for a wave of corruption damage when the effect ends. Especially strong on Decoy and Betty, since both expire by firing, so the corruption wave goes off automatically.

PortalElec+BanditoCactus=Doubled Bandito

Spells thrown through Elec Portal are duplicated, so Bandito's huge wave gets cast twice. Devastating against tightly-packed groups and the easiest way to scale the spell's already-high damage. Every player now carries their own pair of portals (up to 8 in a four-player run), so keeping one open for this is easy.

PortalElec+PistoleroCactus=Doubled Pistolero

Pistolero turret thrown through Elec Portal spawns a second turret on the other side, so two turrets means double sustained DPS until they expire. With every player now carrying their own two portals, you can set this up far more often.