the levels don't make much of a difference, none is eaisier than another, i battle in open level because I got my Latias at level 50, and wanted to EV train it before challenging.
Here's my team that has so far won me every single silver symbol and the tactics and pike gold symbols.
Metagross@lefties
Adamant
252 Att.
128 Def.
128 sp Def
Explosion
Agility
Meteor Mash
EQ
(Basic Agiligross)
Latias@Lum Berry
Bold (was going for Modest but this one has good stats)
252 sp att.
252 spd
6 def/sp def
Ice Beam
Psychic
Recover
Calm Mind
Sceptile@Shell Bell
Modest (bred for max special attack and has a "Wonderfully Outstanding IV total according to the IV man. It took me 3.5 boxes full of treeko to get this guy)
252 sp. att
252 spd
6 sp. def
Leaf Blade (Best Grass Move available other than Solarbeam)
Crunch
Toxic
Dragon Claw
That's About it! I usually start a Battle tower/palace/arena match with Metagross first, Sceptile second, and Latias third, but in the arena Scepy goes last.