I have found that for practice, the zents are the easiest to kill, and the Earth forces are the hardest....thats just me.

I feel the same way, in stand alone you can tell that difference.

heres my recipe for a easy earth kill

2x long range missles b-ss preff
1x micro missle any type higher ranks r better tho
1x Large shaft for the missles he shoot at u
1x impact gunpod rest is up to you

i come from far range shoot 4-5 volleys of LR missles and when i he starts shooting missles i boost up close and use alot of micro missles on him and his dead in 20 sec :p

thats just my strat