they are possibe... if you think that is tough, try the naoya ending, you'll get a surprise final battle. couldn't beat it without suspend and reload
to get gold membership, i think get 250 for your rating... occult needs around 650.
a little tip that carried me through most of the game, try to arm your main char with a monster that has double up, and one with chaos wave. the 4 devas that you have defeated earlier should all have double up, so fuse the one with your appropriate level. chaos wave helps you hit belberith even without raising the platform, which means you effectively can attack him without defeating any of the believers, and his defence is really really low!
for me, i made my main char a caster and by the end of the game his mag stat was 35 LOL! it helps to give all casters an element jump for a large boost in dmg.even with ST resistance against an element, my mc can deal around 250 per hit with a force dance and with element jump; around 450 if they are weak to the element.
if your mc is a meleer, give him phys jump and pierce and life surge for sick dmg with deathbound (it goes to around 450 per hit if your char has around 650 hp). or you could equip this on one of the 4 devas like i mentioned earlier... so it's double up with deathbound! with my set up, i managed to one turn KO belzaboul and belberith (2 battles cos of double up) which saved plenty of trouble
one final tip, if you are dead bent on completing the game at all cost without the use of the cheat database to modify your game stats, here's a trick (which is not as bad as cheating but still cheating to some extent LOL)... save the game suspend data often or before every battle if you want, and when the outcome of the fight isn't satisfactory for you, simply hit the left shoulder+ right shoulder + start +select for a in-game reset and reload. with this, you should be able to defeat the game with minimal grinding and an under levelled party like me.