Certain means sure, definite, so, its antonym is doubtful which means unsure, uncertain, etc.
Philistine refers to a person who is ignorant and guided by material rather than intellectual values. So, its antonym is intellectual; a person who is engaged in the creative use of his intellect and guided my intellectual values.
Irascible means irritable, easily angered, etc. So, its antonym is placid which means calm, good-natured, etc.
Approbate means to approve or sanction officially. So, its antonym is 'condemn', which means to declare wrong, to convict, etc.
'Supercilious' means haughty, arrogant, patronizing. So, its antonym is meek which means humble, timid.