If you're struggling to get into the top two tiers on the standard daily events wait until there is a Delta Daily. These semi-regular community events only have two tiers so by default you finish in the top two.
I had the achievement unlock this way - wasn't expecting it but not going to complain. Be aware though that there is also a separate achievement for finishing in the top tier of a Delta Daily.
The Delta Force achievement in DiRT 4 worth 43 pointsFinish in the Top Tier of a Delta Daily in Community Events