https://royals.ms/forum/threads/ways-to-gain-hp.224740/ Hp washing especially as thief is incredibly expensive and something you wanna do as early as possible (way before level 50 as you are) . There are alternatives in Dave's guide that I think would be better for you at this point!
Also just to add a bit. The bosses that require the most hp are also the bosses that people demand higher range (better equips/damage) for. So by the time your equipment is up to par for being recruited for high end bosses you shouldve gotten yourself quite the hp pool by the "alternative" methods. Happy mapling my friend! An easier way of getting the hp goals for these bosses would be warrior/bucc but dont let hp goals get in the way of playing what u want!
