the celts couldn't beat the Romans...individually or in a group...period.
Attilla vs Alex is actually a pretty interesting concept if you remove the TV (ra ra ra) aspect. They both liked fast flanking cavalry. Attilla enjoyed a scorched earth style defensive posture but lacked the formal training Alex had with formation posturing. Attilla's victories often came against weakened armies and Rome was in a state of internal dissintegration. He didn't enjoy local support like Alex did which makes keeping your army traveling and fed expensive. Alex rarely overextened himself and defeated enemies (Persia) at the peak of their military strength. He also never lost in part because he choose his strikes well. He also avoided the quagmire of Afghanistan.
Attilla isn't really an equitable match. To take on Alex you need a Lee, Patton, Sun Tsu or Hannible. Their styles match up better.