I beat the US Nemesis. It was difficult, 'cause I was never good enough to get some of the ultimate weapons completely unlocked, so I didn't use them. Instead, I just kept double-casting Drain, each doing around 10,000-11,000 damage with Lulu, then keeping everyone in a constant Auto-Life with Yuna, and also using those "Use" items that let you cast magic for free or 1MP (if necessary).

However, I must confess, I used a CodeBreaker to have infinite of all items (never beat Nemesis on my cheatless file before it got deleted...).