Aya first encounters No. 9 as a fight at the end of Akropolis Tower. This fight can be dangerous because there is not a lot of room so the Grenade Launcher and the electrical wires/steam pipe are vital for dealing damage as they can deal 200+ damage to him, which causes his HP to be cut down by a good amount. Pepper Spray will cause PARALYSIS for about six seconds, giving the player a chance to do some free damage. If he gets Aya cornered, his strike will instantly kill her so the player can't prioritize defense over offense too much.
His grenades are easy to dodge if you stand still and they inflict Seizure which can really be a pain, especially as you may not have Metabolism or the ailment cancels the spell. His attacks are not too dangerous unless he corners Aya. Still, he is not too hard of a boss unless you are on Nightmare Mode.
Aya encounters No. 9 after she opens a sealed room after he kills an ANMC. His gear is very different and consequently he has lower HP, but he only takes 50% damage from incoming weapons fire (with the exception of R.Slug). When he has not just attacked, he will guard against all spells, combat items and grenades with the Gunblade, causing the attacks to deal minimal damage. The idea is to get behind him or wait for him to strike then attack. Pyrokinesis (Lv.2 and up) is good as is the Grenade Launcher. His attacks are fairly easy to dodge, provided you keep moving around the room. He isn't too powerful. If you are on Nightmare Mode, you should stick to the corners of the room and trick him into attacking thin air, then let a Pyrokinesis attack rip him up. It should deal 100+ making quick work.