the only way to make a ninja back off is very hard,since you will need to have very powerfull towers to hold him until his 5 seconds of shadow mode is off,then use the speed up spell on all towers to try have a chance against him
to kill the baron,equip disrupt in your action bar 5 or 6 times,when you disrupt him,he damages himself to recover focus,so keep doing this until he dies and if you have some luck you will get his sword
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