Either make the turn limit which is different in each mode or fulfill the alternate requirement listed after the turn requirement. To qualify for 20x, you must have ALL the spheres to make the Shield of Seals, regardless of fulfilling the requirements.
Chapter 3x: Complete in 24(Normal), 28(Hard), 35(Maniac), 38(Lunatic), 43(Lunatic Reverse) or less/1 or less units died/retreated in prologue.
Chapter 6x: Complete in 20(Normal), 24(Hard), 28(Maniac), 32(Lunatic), 36(Lunatic Reverse) or less/Rickard was recruited and is alive.
Chapter 10x: Complete in 15(Normal), 18(Hard), 21(Maniac), 24(Lunatic), 27(Lunatic Reverse) or less/3 or more enemy clerics survived.
Chapter 13x: Complee in 20(Normal), 24(Hard), 28(Maniac), 32(Lunatic), 36(Lunatic Reverse) or less/Both enemy theives didn't escape the map.
Chapter 16x: Complete in 18(Normal), 21(Hard), 25(Maniac), 28(Lunatic), 32(Lunatic Reverse) or less/My Unit is promoted and Level 5 and up.
Chapter 20x: Complete in 24(Normal),28(Hard),33(Maniac),38(Lunatic),43(Lunatic Reverse) or less/3 or more of Wolfguard and Midia joined and lived.