If these are top-level events, then "For each DummyAttack" is triggered on every tick, but you have another condition "Every DummyAttack.attackSpeed seconds".. Not only this is not good for performance, but I can't imagine how these two conditions work together, especially if different instances have different attackSpeed values.
Use Timer behavior instead. When DummyAttack is spawned, start its Timer. In "On timer" event do "For each DummyAttack", pick Enemy, subtract from health.