Logical steps against nuclear enemy would be:
a) Arms race
1. Building up your nuclear forces to a level in which enemy would be destroyed in any possible scenario should nuclear weapons be used, thus making enemy unwilling to use nuclear weapons.
2. Developing defence systems capable of destroying enemy missiles or whatever delivery systems they might be using thus nullifying enemy nuclear threat.
Both of the above would be extremely draining to economy and could lead to other serious problems (compare to the collapse of USSR).
b) Political pressure
1. Economic sanctions. China has huge influence on world economics and can use that to hurt her enemies.
2. Other sanctions. China could push for UN Security Council to make resolutions in order to contain the threat, eg. isolating the enemy from rest of the world.
c) Alliances with other nations
1. China is already a member and a driving force in SCO and in case of being seriously threatened by nuclear enemy Chinese government would probably seek assistance from the alliance.
2. Other minor alliances, mutual defence pacts &al., could be made with any country with relevance.