Leafeon or Abomansnow.
Leafeon can be obtain by leveling up Eevee near the mossy stone in eterna forest. With its 110 base power of attack and acess to lead blade, it can beat the elite 4, except maybe Flint
Leafeon
Adamant
Your choice of item
Leaf blade-STAB
Swords Dance-Setup
Synthesis-HP recovery
Last Resort/X-Scizzor- 2nd attacking move
Abomansnow can be obtained by evolving Snover, which can be caught near Snowpoint city. Has decent stats in everything except speed, but an ally that can use trick room can fix that, such as Dusclops.
Abomansnow
Brave nature
Your choice of item
Wood Hammer-STB
Ice shard-STAB, priority
Ice Punch-STAB
Sheer cold/Avenlache-To help out in situations
Hope I helped!