So to be the more effective you need to have finished the main quest.

Then it is really simple, you have three heroic events every days in Huaixiu village ,Baixan Plateau and Shanguai Grassland (1 by zone) that gives you three intermediate orders, so you can earn a total of 9 a day from what i've found.

So you can approx increase 3 levels every 2 days on 1 companion.

Have fun !