For the most part, you can sit with one of these cards on the battlefield for most of the game and wait for them to cast a Past in Flames before going to exile their graveyard, but if the Past in Flames is in the graveyard already, you may want to preemptively go for it
If enough -1/-1 counters are put on Hei Bai at the same time to make his toughness 0 or less, his last ability will see all of the +1/+1 counters he had when he left the battlefield as well as the -1/-1 counters he had, and an equal number of each of those types of counters (plus any other applicable counters) will be put onto the target creature
One mana is a steal, however, as shown by cards like Blasphemous Act
If the drawn card is a magic card, that player cannot Normal Summon/Set it that turn
Also, protect your cards, as their price will directly depend on the card's condition
Possibly most famous are the Ojamas, which have 0 ATK and only 1000 DEF, absolutely silly and ridiculous card art and are treated like Butt Monkeys in other card art, but with their support cards they can be very dangerous