Itachi SA has fixed damage amounts (the fire damage first round and sasuke's attack last round) in their damage calculation. It seems to be something like 500 damage + level scaling damage first round and 2500 damage + scaling damage in last round. Taking 2000 damage first round isn't a big deal when you have 15000 HP, but when you are low level you can't even survive with good shields. Your best bet is to get help from someone who is a higher level. If you are on a new server, then you are just out of luck until next week. The fixed amount does get slightly smaller on normal, but on easy it is either 0 or very very small.