Right, but you have a target. A square. If you want to ignore the rules fine, but in that case this isn't the forum for you. Unless you can somehow demonstrate that pg 272 doesn't say what it does say (And no, it isn't the section on attacking invisible creatures, its the rules on attacking anything).
Actually, I think the bit about requiring a target is a red herring. The "Target" entry on page 218 of PH2 states:
If a power directly affects one or more creatures or objects, it has a "Target" entry, specifying whom and what the power affects.
In this context, the "Target" entry is informational, not prescriptive. The "Target" entry tells you what the power affects directly; it is not a requirement for using the power.
Furthermore, the text of "Legitimate Targets" on page 40 of DMG1 states:
When a power has an effect that occurs upon hitting a target - or reducing a target to 0 hit points - the power functions only when the target in question is a meaningful threat.
Hence, it would appear that the DMG1 prohibition is only against allowing the characters to enjoy the benefits of "Hit" entries, or any benefits that trigger when a target is reduced to 0 hit points, when the target is not a legitimate threat. Most "Effect" entries do not fall into either of these categories.
In fact, the description of "Effect" on page 219 of PH2 states:
Anything that appears in an "Effect" entry occurs when you use the power, whether or not you hit with it, if it is an attack power.
Since the "Target" entry does not mean that you are required to attack a target in order to use the power, and since the DMG1 prohibition is against enjoying the benefit of "Hit" entries and not "Effect" entries if the target is not a legitimate target, and since the "Effect" entry of a power occurs when you use the power, regardless of whether or not you hit with it, if it is an attack power, I would say that you can gain the benefit of the "Effect" entry of a power without needing to attack a target, even if the power has a "Target" entry of "One creature" or "One enemy".