I've solo chained with a 23-25 cnj, 25 pug, 34 mrd, and 47-50gla. You just have to be very efficient with your abilities and have a good action bar setup for solo. Anything more than 3 is very difficult solo because of the time contraints. Also +3 levels above current level seems to be about the threshold for when fights become difficult/inefficient particularly because monsters start to evade your attacks more and crit you more often.