You hire a certified arborist because that person/company has the appropriate equipment.

Do it professionally or let a professional do it.
Spraying overhead has many disadvantages, almost all to the operator. If you cannot direct a chemical downward and away from you, let a professional do it. If you have gone through the steps and you have determined that you must intervene, with a chemical, and you have chosen a contact chemical...then it probably involves a 'cherry picker' type of truck, PPE etc. On the other hand it may be possible to use a systemic which can be applied to the roots and now, it may fall into the realm of a DIY project.

Non toxic scale remedy

Here is a link that might be useful: Scale

kkfromnj's link brought out a point that ants may be contributing to your pest problem. Do you see a trail of ants climbing the tree? If so just stopping the ants (easier said than done) could make a big difference.

The information about the ants was largely incorrect, however. Scale insects can and do develop large populations without any assistance from ants whatsoever. Merely controlling the ants that are taking advantage of the honey-dew producing scale insects will do nothing to control the real pest.....the scale.