As this is a thread asking about game content, rather than an account issue, it has been moved to the RuneScape Guides and Help Forum for best results.
My favorite way of training combat skills has been through slayer. The variety of the tasks keeps it from getting boring or feeling like a grind.
I would say that repeatedly killing your alt in Clan War white portal gives pretty great xp!
You just quickly chain build and keep high adrealine and spam the strongest combat abilities and then your alt just enters the same area again to being killed by your main!