Guts will contradict with Lum Berry.
Guts is only in effect whilst the Pokemon is suffering from a major status ailment, curing the status will thus remove the boosted attack.
Flavor text from the games:
Generation III : Ups Attack if suffering.
Generation IV : Boosts Attack if there is a status problem.
Generation V: Boosts Attack if there is a status problem.
Generation VI: Boosts the Attack stat if the Pokémon has a status condition.
Attack is increased 50% when afflicted by a non-volatile status.
Our own dear Pokemon DB:
Guts raises the ability-bearer's Attack by 50% when inflicted by a major status ailment (poison, burn, paralysis, freeze or sleep).
So in short, Guts only works when the Pokemon suffers from a status problem.