Guardian has 2 phases to fight and they are very different requiring slightly different strategies.
There are plenty of videos on youtube on how to fight every boss but I always find a written guide a bit more handy for little tips and such.
Things to equip - Healing Gourd/pellets (obviously), Purple gourd or pacifying agents, Antidote power. Firecracker, Flame vent and Thrust spear prosthetic
Stage 1 - This is a quite a standard fight and really easy/quick if you can get the deflect timing down.
-They can be stunned with the firecracks a couple of time but make sure you space out your usage to avoid waste, same goes for the flame vent.
-They have 2 poison attacks but if your bar is empty you should be safe from getting poisoned.
- 2 grab attacks one where he tries from the left that can be jumped and leaves them open for hits and the other where he jumps at you and is a pain in the arse to dodge.
-Everything else can be deflected for posture damage, if you succeed in deflecting there are big gaps in the attacks open for punish and you will notice if you do enough damage he stops moving, KEEP ATTACKING, he can be stunned like this 3 times in a row before running away so make the most out of this time.
-Best advice about this fight applies to most bosses, keep the pressure on and don't let the posture bar start to drop.
Stage 2 - This part is a bit more frustrating, for me at least. Deflecting is still the most important aspect as always. His moves are more unnatural which might throw your timing out slighlty, but every move has a decent enough wind up,
- Your fire no longer works now the boss is headlesss so make sure your spear is equipped.
- The hardest aspect of this fight for me was his terror scream, If you see the Ape lift his head to his shoulders RUN. If your still close to him when the red character appears above your head then the Terror is going to hit you. If you're not familiar with Terror, if that bar fills you are dead.
- The best moves to watch out for is his flurry which starts from a standing position, if you deflect all of them the last move should be an overhead slash, deflect that and he should fall to the ground. Thrust spear his neck hole R2 R2 and you should rip out the bug keeping the Ape alive dealing massive posture damage. In my successful run I had to do this 3 times along with standard deflections before finally getting the Shinobi execution