u can use any units u wish for xerxes, but these very expensive units as mentioned will fill bank twice as fast to get big payout: pikeman, golden shield, cartharians (numids are expensive as only one defensive variant but not as expensive as pikeman)
there are no other defensive expensive units, but u can use any other normal units, but preferably lower tier as u better to keep MPs and stuff for pantheons etc
or u can simply just load positions with offense and when u close to payout, then kill 1 defensive position with defense to get defensive reward
if u play offense, u should attack with unit types that xerxes has smallest amount of defense vs them. also click on xerxes units to see their increase defense vs same type unit. I think horses will have like 80 defense vs your army, but 240 vs your horses, its goes to each class of units, just click on his units and have alook.
if u do defense its easier, xerxes army has simple offense numbers, if for example most of his army is light infantry its bad idea to defend with MPs as u know MPs have low defense vs lights, in this case click on your own units and choose ones who will counter xerxes units best way, focus on units which he has most off.