yes i see several weaknesses, the main one being gyara. i know you're not going to like this idea, but drifblim really needs charge beam or thunderbolt. no STAB, i know, but it's where drifblim really comes in handy. regardless, you can switch drifblim into gyara. if it uses dragon dance, it won't KO drifblim on the next turn which gives you the option of attacking it for the 2HKO or putting it to sleep after a charge beam or thunderbolt, hoping that they will switch out. drifblim doesn't need shadow ball anyway. i know gyara is OU tier, but if you plan on using this team in competitive battling, it's definitely something worth considering.
next, another problem with drifblim: salac berry makes no sense. no one on your team will really benefit from the speed boost. also, only ONE of your pokemon makes full benefit of Calm Mind + Baton Pass -- octillery. it doesn't even need speed, so again, salac berry is pointless. i highly suggest you replace salac berry with either Petaya Berry or just plain old Sitrus Berry. petaya berry will boost sp atk, which will help octillery (or maybe even meganium), and sitrus berry is so that drifblim doesn't get owned by shadow sneak/ice shard.