Stringent means severe, strict, harsh, so its antonym is lenient which means soft, merciful.
Major means main, chief, key, so its antonym is minor which means small, negligible, and unimportant.
'Essential' means vital, important, necessary, so its antonym is unnecessary which means needless, avoidable, etc.
Hypocritical means insincere, duplicitous, and two-faced, so sincere is its antonym.
Fickle means inconsistent, changeable, so its antonym is steadfast which means persistent, committed, etc.