Augment, item and mechanic interactions most players miss in ARAM: Mayhem.
With Karthus's 
RequiemR or Ashe's 
Enchanted Crystal ArrowR, 
From Downtown calls down giant meteors like Ziggs's ultimate. The downside: the meteors fall rather slowly.
Turn on 
Lightning Field / Hyper ChargeW in cannon form before your 
To the Skies! / Shock BlastQ + 
Thundering Blow / Acceleration GateE combo. 
Purist - Caster converts the attack speed from W into Ability Haste, so your cooldowns drop massively.
Javelin Toss / TakedownQ deals more damage the farther it flies. With 
Spell Split, the split spears keep the distance already traveled, so they hit as hard as the main one.
Shield VaultW deals damage based on Pantheon's max health. Stack enough health with 
Tank Engine and 
Steel Your Heart, and one W can deal damage equal to the target's entire max health, or more.
Tongue LashQ grows as Tahm Kench grows, so 
Goliath and Tank Engine fit him perfectly. With 
Ethereal Weapon too, rush 
Dusk and Dawn: one Q can add up to 3 passive stacks, plus damage and healing.
