let's try to make some calculations!
hilltop fully fortified archer in a city with 20% defence is the toughest thing your Quechues can face (unless you fight agains Hammy/MM/Protective, attacking those last is situational)
25(hill)+25(archer on a hill)+50(city)+25(fortify)+20(culture)=145%
-100% for quechues uniquity, and the archer still gets +45%
let's assume you did not build a barrack, although i'd advice to.
then it would be 2.2 vs 4.35.
It' something like grave odds, you need like 4 quechuas per an archer to be sure, and the capital is usually defended by 2 or more. Assuming you did not built barracks, you attack early and it is 2 archers.
if the city is not hilltop, combat is 2.2 vs 2.85 (this is still 3 quechuas per archer).
So, attacking with 5 quechuas does not seem right, you need a minimum of 6 even against non-hilltop cap, and 8 or more in your case.