"Elemental HERO Sparkman" + "Elemental HERO Clayman" Must be Fusion Summoned and cannot be Special Summoned by other ways. Once per turn: You can discard 1 card to target 1 face-up monster on the field with original ATK less than the ATK of this card; destroy that target.
Most high level monsters that are often used are at around 2400 attack or higher, so you'll need to either lower their attack, or increase Thunder Giant's attack. Activate and prolong Mirror Wall when your opponent attacks your other monster, then fusion summon Thunder Giant and activate its ability. You can discard cards like Skull-Mark Ladybug, to recover some LP, or Needle Sunfish to lower a different monster's attack.
Discards for cost
Destroys face-up Monster Cards
Nomi / 2 Fusion Materials
So far I'm not having any luck pulling this or another Armed Dragon Lv 5 from duel rewards. I've gotten several Neos, Ojama Kings, and a Tempest, though.