for the last boss use matts stone edge. buff ur def,mdef and atk as often as possible and heal every single round. concentrate ur attacks only on the base, the cannon and the guy arent importand. use full skills are counter,the lucky star counter, random airstrikes and even the random nolegs (he can be a lifesaver!) u will need lots of ethers an water of life cause natalie dies pretty fast... and some luck could be usefull too...
for the hydra u need a bit luck. at first blind it with flare and then attack with holy and fire. try to kill both heads at once because 1 will allways revive the other. actually it can happen that both heads use that insta death skill and u lose but that doesnt happen very often.... soo i hope that will help^^