I have been an aikido practitioner for only a year, however, I did practice jujutsu almost two decades prior, so I am not sure if I am qualified to offer a suggestion, but I'll try.
Giovanni has the right idea of "not being there." Movement to avoid the initial attack prior to a counter would be my first choice.
He is also right (if I am interpreting him correctly) in the fact that sometimes you don't have that option.
All martial arts have solutions to the situation you described with Aikido being no exception. For example, if the attacker is running at you like a freight train (my interpretation of your description of "centered, hard, and tense"), aiki nage might be one of many good choices.
Your choices may be limited to your ethical beliefs. Again, as Giovanni questions, what are you trying to do with the atemi? I believe there are some people on this site who believe that the use of strikes(especially to create physical trauma) would violate the standards of the art. If you hold this belief, atemi may not be an option for you.
If would help if you could describe an example of the type of attack you're talking about. I feel I could be of more help.
With best regards,