Grab a low level player that can do the H2, dont form a group yet, have him summon the champions because they always have the level of the highest lvl players in thee group but when solo it will have the level of the player, have him start attacking the champion and THEN form a group. Whos inviting who doesnt seem to matter. Now you can attack the champion who is lower than your level and has not as much HP as at higher levels. At lvl60 they have roughly 150.000 HP, at lvl70 its about 1mil. So if you dont have that much damage with you or a low level, is is very handy.
Unfortunately this doesnt always work when attacking the rakghouls for the second part of the H2 mission.