I say you re-train a whole new team. Get a BIG mix of Pokemon types. You can even EV train them, but you might not want to do that.
A team I would suggest for Battle Frontier is:
Swampert @ Leftovers
EVs: 188 HP, 176 Att, 144 Sp. D
Nature: Brave (+ Att - Spd)
Abillity: Torrent
-Earthquake
-Curse
-Rest
-Rock Slide
Swampert need sDef EVs, even with curse.
Great wall and physical attacker. This might also go in Strategies and Movesets section, but I'll answer anyway. Swampert's one and only weakness is grass, the special defense EVs cover it's grass weakness since most grass attacks are special attacks.
Most everything will still OHKO it ._.
Slaking @ Leftovers
EVs: 252 Att, 154 Def, 104 Spd
Nature: Adamant (+ Att - Sp. A)
Ability: Truant
-Hyper Beam
-Slack Off
-Amnesia
-Shadow Ball
the only good slaking set I've seen is the CB one.
Slaking basically OHKOs everything with Hyper Beam, and since he has Truant ability it ignores the recharge, and he just loafs around anyway. With the Amnesia and 154 Defense EVs, Slaking will Tank A LOT. Slack Off will heal you and make Slaking reckless. Shadow Ball to take out the ghosts who will resist Hyper Beam.
slaking cannot tank, it's that simple. traunt ruins that and you'll be KOed by special attackers anywaya. Its purpose is to hit and run, not the opposite.
Milotic @ Leftovers
EVs: 148 HP, 104 Def, 56 Sp. D, 200 Sp. A
Nature: Calm (+ Sp. D - Att)
-Hypnosis/Toxic
-Recover
-Surf
-Ice Beam
Milotic is a great wall and special attacker. With this team, your whole team is basically walls and can withstand attacks, while still being great attackers. All can heal themselves too, so make great use of that.