You don't choose a target for The Ancient One's activated ability at the time you activate it
Just make sure to keep the cards that you draw separate from the rest of your hand until after you resolve the upkeep trigger
While itll take some reps to understand where and when you can go off, the how is relatively easy: make a bunch of mana with rituals, cast cantrips to run up the storm count, mix in a Past in Flames, and then put a lethal number of Grapeshot copies on the stack
If you do run into any problems, please use the bug report link or contact customer service
But even so, it can still be a fun self-imposed challenge when you need a bit of a break from your tier 1 meta deck
Mech Hangar does something similar on a land, but it can only do this to one vehicle per turn (if you arent untapping it somehow)