In a normal situation, the Wurm would deal 15 damage to the Colossus, and with only 11 toughness the Colossuss health would be reduced to zero, sending it to the graveyard
Seasoned Dungeoneer Dont get me wrong, the main reason Seasoned Dungeoneer is at the top is the initiative mechanic, which is a busted mechanic in many formats
Furthermore, when it uses this effect, it cannot be destroyed by card effects until the end of the opponent's next turn
halve the ATK of the opponent's monster, and if you do, this card gains that same amount of ATK
Whenever Squall deals combat damage to a player, return target permanent card with mana value 3 or less from your graveyard to the battlefield