Archers needed: Level 1- 40
Level 2- 75
Level 3- 150
Level 4- 375
Level 5 and above you will start to have casulaties. Here I recomend sending at least 700 per wave. But split them up to each hero( Ex. 6,000 Archers could be split up to 1,000 per hero, if you have 6 heros).
I used +700 archers to take a lvl 4 valley, and I still had losses...