It's been a while now since I've played FF4 or fought Octomammoth, but I don't remember him as being a particularly hard boss. You should look at some guides on the FF4 ds board but here are some tips:

-Use the spell slow on him as soon as you get the chance, this is a very useful spell for bosses in this game

-Cast lightning on him, as he's weak to it, have Tellah do this

-Make sure to have Tellah and Rydia in the back row and constant healing from Rydia(Tellah also if needed)

-Take advantage of Cecil's Darkness ability but then also make sure that his hp doesn't drop to low

-If you still can't beat him than try grinding, or maybe someone else will have a better idea

Keep trying and good luck! ;)

notyalemblem

Thank u for the helpfully it will work cause i died 2 times trying to kill him tyvm

*sigh*

plzzzzzzzz help me