Hold SHIFT when clicking on mana pool icons to use as much of that mana type as possible towards the cost
Furthermore, it is unknown if there were any conditions that prevented or dissuaded "Dark Magician Girl" from activating her Deck Master ability earlier in the Duel, for repeated uses
If youre about to be killed by a gigantic Craterhoof'd flunge and you plop this down only for another opponent to bounce it back to your hand, then you really should have been playing some sort of fog or protection spell, which not only are more difficult to interact with but also much, much cheaper
The first roll costs 0, the second costs 1, the third costs 2, and so on
This effect gives you a bit more freedom to strategize, especially if you know what you are playing against