|first thing you need to know is:
you should send challenges from the char you want to drop the exp from, the recipient always hits first.
second thing you need to know is:
you can only send challenges to chars 10% or more of your own level, for instance level 1,000,000,000 can challenge level 100,000,000 and higher.
third thing you need to know is:
stats, you need to work out how to beat your main char, taking into account its defence, magic shield, attack rating and magic rating, it's attack spell and weapon damage are irrelevant.
you need an attack that can beat your main chars lowest defensive stat (defence or magic shield) and an adequate amount of attack rating or magic rating depending on which school your helper is going to be in (melee or magic attacks)
building a helper chain without borrowing some exp from someone elses helper chain will be very time consuming, it would be easiest for you to trade someone some exp from your main char for some exp on a few lower level chars to get the first few chars in your helper chain made