Timestop is certainly sketchy perhaps even broken but it can be counterspelled by the dragon.
Haste is a massive help but literally there is no way for the PC to keep up enough of an assault to actually kill the Dragon short of a insta-kill spell.
In terms of Harm, while this is a massively broken spell a forbidance in place over the dear dragon's lair makes getting to him in order to do a touch spell somewhat sketchy.
Gate is realistically the only possibility for the dear wizard but fortunately for the dragon forbidance in the lair makes for a very very effective barrier to planar travel
Haste is a massive help but literally there is no way for the PC to keep up enough of an assault to actually kill the Dragon short of a insta-kill spell.
In terms of Harm, while this is a massively broken spell a forbidance in place over the dear dragon's lair makes getting to him in order to do a touch spell somewhat sketchy.
Gate is realistically the only possibility for the dear wizard but fortunately for the dragon forbidance in the lair makes for a very very effective barrier to planar travel
