Whenever Avalanche Tusker attacks, target creature defending player controls blocks it this combat if able.
9/20/2014 If the target creature can’t block Avalanche Tusker, but could block another attacking creature, it’s free to block that creature or block no creatures at all.
9/20/2014 If, during the declare blockers step, the target creature is tapped or is affected by a spell or ability that says it can’t block, then it doesn’t block. If there’s a cost associated with having that creature block, its controller isn’t forced to pay that cost. If the player doesn’t, the creature doesn’t block in that case either.