For that, i just used the 10% exp to lvl them
or what you can do is reduce hp of monsters by using ur other hero,
then use the skill of leesunshin to lvl him up.
For sinlip, u can get a calivier thats high lvl, then equip him new power bow or military bow
then use skill just like leesunshin. What i did is just leech them. its not hard.