Thanks for all the hints and tips! Actually I haven't tried legendary again after I buffed up my team, and when I got back suddenly everything went smoothly XD
Therels trial and error between my try, my tips are :
1. Learn the attack pattern of the enemies, and defeat the enemies in order <= this is very important
2. Not bringing champion and use shadow/ranged heroes will help
3. Use the attack buff item on archer/shadow to kill the most dangerous enemy first
