Amarant is the only one who can do it, and you'd need to get him to 84 Strength.

Multiple items that teach the same ability don't affect the strength of that ability. That is to say two pieces of equipment that teach HP+20% will not give HP+40%.

Matter of fact having both HP+10% and HP+20% doesn't give 30%, it gives slightly more. Max Hp is given a 10% increase first, then Max+10% is given a 20% increase.