Try the overdose method of 5ml of Metricide 14 per 10 gallons for a 10 - 14 day period. Misting the algae during "low tide" during a waterchange with a 1 part Metricide 14 and 20 parts water will also give the BBA kill a head start before beginning the 10 - 14 day treatment.
extend the reach of it then, you can use airline tubing on a syringe, measure into a container before hand so you don't over do it. If you need something more rigid to aim the tubing, straws, skewers or sticks tied to the tubing with whatever is tank safe. just make sure the filter is off for atleast 20 minutes so where you treat stays concentrated longer, otherwise flow will dissipate it too quickly