I was messing around with my brother's elite controller earlier because I am debating whether or not to get one and practising against the poor raider bot in how to play (I have beaten him up so often, revenge for the twenty times in a row he would kill me before I could score a win at the beginning) and my fingers were everywhere pretty much but I did something by accident I have never thought to do before, I went from deflect to guardbreak.Is the guardbreak guaranteed from this situation and is it a better option than light attack?

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I answered my own question against bots, it isn't guaranteed, but odds are it isn't something an opponent will expect either.