It depends on which form of tackling you are using:
verb, is the correct answer in the case of "while tackling his homework inside"
A verb is a word used to describe an action or occurrence, "tackling" in this case, is an action that Robert is "tackling" his homework.
adjective, is the correct answer in the case of "the tackling dummy."
An adjective is a word used to describe or modify a noun, which is exactly what "tackling" does in modifying the noun, "dummy."
