Luckiest Spider may assign its full combat damage to up to 8 creatures blocking it. If there are fewer than 8, then to all creatures blocking it and defending player.
Luckiest Spider can't be the target of spells or abilities unless their controller pays an additional 8.
See Challenge # 073 for Challenge # 051 and Challenge # 102.
So, a 12 mana 8/8. Or, I guess, a 13 mana 9/9 or a 12 mana 16/16...