Walking Ballista
Artifact Creature — Construct
Walking Ballista enters the battlefield with X +1/+1 counters on it.
: Put a +1/+1 counter on Walking Ballista.
Remove a +1/+1 counter from Walking Ballista: It deals 1 damage to any target.
Illustrated by
Aether Revolt Promos#181sRareEnglishLegality
Rulings
A casting cost of means that you pay twice X. If you want X to be 3, you pay 
to cast Walking Ballista.
If Walking Ballista has been dealt damage or had its toughness reduced by an effect, this limits how many times you'll be able to remove +1/+1 counters from it in a single turn. For example, if it has three +1/+1 counters on it and has been dealt 1 damage this turn, it will be destroyed immediately after you activate the ability a second time and you won't be able to activate it a third time.
A casting cost of means that you pay twice X. If you want X to be 3, you pay to cast Walking Ballista.
If Walking Ballista has been dealt damage or had its toughness reduced by an effect, this limits how many times you'll be able to remove +1/+1 counters from it in a single turn. For example, if it has three +1/+1 counters on it and has been dealt 1 damage this turn, it will be destroyed immediately after you activate the ability a second time and you won't be able to activate it a third time.