Rules of the 1 Life Cube:

In order to emulate starting at 1 life in Arena, any player with 19 life loses the game as a state based action.

Effects that "double" or "halve" life are excluded.

No 1 mana creatures that can attack on their own.

2 mana creatures that can attack must have a cost restriction or serious downside - double pip mana, needs another card or effect to allow it to attack, etc.

Direct damage outside of combat requires 4+ mana or additional set up.