The abilities a leveler grants to itself don’t overwrite any other abilities it may have. In particular, they don’t overwrite the creature’s level up ability; it always has that.

Effects that set a leveler’s power or toughness to a specific value, including the effects from a level symbol’s ability, apply in timestamp order. The timestamp of each level symbol’s ability is the same as the timestamp of the leveler itself, regardless of when the most recent level counter was put on it.

Effects that modify a leveler’s power or toughness, such as the effects of Giant Growth or Glorious Anthem, will apply to it no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness (such as +1/+1 counters) and effects that switch its power and toughness.

If another creature becomes a copy of a leveler, all of the leveler’s printed abilities -- including those represented by level symbols -- are copied. The current characteristics of the leveler, and the number of level counters on it, are not. The abilities, power, and toughness of the copy will be determined based on how many level counters are on the copy.

A creature’s level is based on how many level counters it has on it, not how many times its level up ability has been activated or has resolved. If a leveler gets level counters due to some other effect (such as Clockspinning) or loses level counters for some reason (such as Vampire Hexmage), its level is changed accordingly.

The prevention effects prevent 1 or 2 damage, as applicable, from each source (regardless of who controls it) each time that source would deal damage to you or a creature you control. They prevent damage of any kind, not just combat damage.

Remember that Hedron-Field Purists never has both of these prevention effects at the same time.

If a source would deal damage to multiple applicable recipients, the prevention effects apply to each of those recipients separately. For example, if Hedron-Field Purists is level 1 and you cast Earthquake ("Earthquake deals X damage to each creature without flying and each player") with X equal to 3, that Earthquake will deal 2 damage to you, 2 damage to each creature without flying you control, 3 damage to each other player, and 3 damage to each creature without flying other players control.

The prevention effects don’t affect damage dealt directly to a planeswalker you control (such as combat damage, or damage Sarkhan the Mad deals to himself due to his first ability). They can prevent noncombat damage that’s redirected from you to a planeswalker you control if you apply these effects first.

The effects from multiple Hedron-Field Purists are cumulative.