When Selfless Squire enters the battlefield, its first ability triggers, preventing all damage that would be dealt to you this turn.
Cast Wheel of Misfortune by paying ({2} magic symbol) ({R} magic symbol) , selecting an arbitrarily large number, and dealing that much damage to you if you chose the largest number, which is prevented due to Selfless Squire.
Selfless Squire's second ability triggers, putting a number of +1/+1 counters on it equal to the damage prevented.
If you didn't choose the largest number, Wheel of Misfortune deals damage equal to that number to each opponent who chose that number.