Whether it works or not, ELSEIF is the correct format.
And, no. You don't need the parentheses. It doesn't hurt to have them, though. I prefer seeing the parentheses around the whole IF so you can easily see where it is. The inside parentheses aren't necessary unless you're trying to change the order of operation:
if (%this == 100 && (%that == 1 || %that == 100)) { echo -a test }
Otherwise, it checks the && first, then does the ||.