Berserker's Frenzy

Dice used must have equally likely outcomes and the roll must be fair. Although physical dice are recommended, digital substitutes are allowed, provided they have the same number of equally likely outcomes as specified in the original roll instruction.

If there's a cost associated with having a creature block and you choose for that creature to block, its controller can choose to pay that cost or not. If that player doesn't pay that cost, you must propose a new set of blocking creatures.

Some abilities, like that of Pixie Guide and Barbarian Class, replace rolling a die with rolling extra dice and ignoring the lowest roll. The ignored rolls are not considered for the effect that instructed you to roll a die, and do not cause abilities to trigger. For all intents and purposes, once you determine which dice count, any extra dice were never rolled.