Pattern

Why not start x at 2 and use (60+18x)/x -> 18 + 60/x?

Taking into account the player as a target that makes it x=#targets(incl. players) -> 18+60/x per monster

Then we should have 78 first in the sequence :x

but sure:
x = 0 = first element
(96 + 18 * x)/(2 + x)

x = 2 = first element
18 + 60 / x

x = 3 = first element
(42 + 18x) / (x - 1)
or (18x^2 + 60x + 42)/(x^2-1), though that looks rather horrible