I beat Ninja Gaiden 1 and 2 when I was younger, and got really far in 3, but I could never beat the level of spikes. The games were hard, but the main trick really was to just keep going forward constantly, that prevented the enemies from respawning. Its when you hesitated too long and they started re-appearing outside of their programmed pattern that they became impossible.
Part 2 was acually a lot easier than part 1 because while climbing became a simple "press up" as opposed to repetitive jump, and you had shadow clones you could position to fight for you that were invincible, and made some of the bosses a lot easier.