when you say top nut are you talking about the one that retains the strut assembly in the car or the one that retains the spring?
when removing the one that retains the assembly in the car you don't need the allen wrench, just a 18,, socket and impact if you like.
to remove the nut that retains the spring you may need the allen wrench. if the strut is shot just use vice grips on the strut shaft and use an impact to remove it, once spring compressors are in place. on assemble you might need to use the allen to hold the strut shaft from sprinning while tightening the nut.