In the competitive scene the viability of Elemental HERO has been mixed, with the deck and cards from the archetype ranging from irrelevant to defining the format
Main Deck Bigguns With your Deck consisting primarily of Level 3 DARK Fiends and Traps, you need some large bodies that are easily Summonable to help protect your Life Points and/or provide other benefits such as being a large wall in general, something to help you with follow-up plays, removing threats on your opponent's board, and protecting your own board
It's mostly for recycling the Link Monsters, especially Kagari since Master Duel has it limited to 1 but I like to summon it early on to push for an early-game advantage, though I do also use it to recycle other hand traps (but often it's stuff like Skull Meister or Crow since those cards are incredible vs Eldlich and other grindy matchups)
You can only use each of the following effects of "Ravenous Crocodragon Archethys" once per turn
15 Swallow's Nest Though this card is only suited to players running decks with Winged Beasts, for those who are Swallow's Nest is a hugely powerful option
When To Go Into Extra Deck Monsters