Of course, you'll want some haste enablers in play to do essentially make Old Gnawbone free, but the creatures you already have in play can generate Treasure as well
For example, if you play a Mountain and your chosen number is 2084750930, you'd only be able to cross out one of those zeroes
So, what are battles
It can't be combined with other alternative costs, such as casting a spell "without paying its mana cost." (2018-07-13) To determine the total cost of a spell, start with the mana cost or alternative cost (such as a miracle cost) you're paying, add any cost increases, then apply any cost reductions
The best is Brainstorm since you draw three and then return two, so it's strong with putting back lands, expensive stuff, hiding from discard and reshuffling with fetch lands et all