after defeating d e4,look all over hoenn until you see him but you will need the ff:
1.trapinch-he prevents escape coz of its arena trap.you find him in the dessert.
2. wobbuffet-he also can prevent escape with shadow tag so you could just use him than trapinch.
3.timer balls-use it incase it becomes a long fight.find it in rusborto
4.ultra balls-just incase u run out of timer balls.
5.many hyper/max potions-just incase many pokemon faint.
6.high lv. trapinch or wobbuffet-they will get easily killed if they r weak.
7.patience-its gonna take a long time before u can catch him.
8.save-save after catching him
if u ask y i put this:latias hates fighting and fleds if u damage him so just quickly use timer/ultra ball before he gets away.