I'd go with 60ft diameter balls instead, to benefit from the obscene bonuses to attack rolls they'd get. That cuts you down to only 16.7 million legion devils, but if they're deployed in 60ft square groups, each group collectively has 2736 HP, is guaranteed a natural 20 on any mind-affecting spell or AoE spell, and has a +580 bonus on attack rolls (for a pitiful 1d8+3 each, but hey, you can't have everything).<br/><br/>You also get 116 thousand of these squads to command, with 144 devils per squad.