You should go for a balanced team, an all Offensive team can be easily destroyed by tanks/defensive pokemon.
Actually, offensive teams are by far the most common types of team in the current metagame. Numerous attackers are powerful enough to break walls nowadays.
darthalex, an offensive team consists of fast hitting sweepers and revenge killers. There are numerous options for an offensive team. Obviously, it needs to be balanced to take advantage of resistances and such, but that's the general idea ;D