six months may sound a very short amount of time but if you capitalise your time then it won’t be a problem.
Create a 24×7 schedule (worked for me) and work on the difficult subject first. Physics and Chemistry should be worked on first. Brushing up 11th topics like mechanics, waves, SHM, fluid dynamics, etc. Chemistry chapters of Organic chemistry and physical chemistry are of paramount importance since these are concept building chapters.
12th Physics and Chemistry should be done next with focus on electrostatics and magnetism. Nuclear physics and semiconductors are equally important. Chemistry topics on physical chemistry(12th) has been put in the paper all the time. Oraganic chemistry is the most important topic of 12th so leave no stone unturned.
Lastly study biology since it is the easiest and takes least time amongst all the subjects.