it sure is, especially the Sparta level, to easly beat it, just buy the laser cannon and upgrade it to the maximum, and just fire everywhere, it's really easy
too hard? dunno about that, had a couple glitches but other than that I was able to beat the campain on my second attempt. first attempt I ended up with 3 invincible tanks due to the glitch during day 39, I wasn't pleased.
Anyway... it's pretty simple, 2 maxed shield towers, 1 maxed missile tower, maxed BFG (Old Glory), maxed S.C.A.R. (during early lvls) spend the rest on repair guys and maybe 20-30 shooters. Once you get to day 35+ just be sure to concentrate on tanks and walkers, when they're not present just help the shooters clean up the fliers. At day 51 I had 77 repair guys and 45 shooters (over did it on those). I hadn't spent any money on other weapons, towers, or even repairs... the only reason I didn't move past that day is because I got tired of playing :P I had 582 life at the end and had serious carpel tunnel syndrome going lol
oh and I forgot, as far as I could tell, the glitch was caused my any weapon that did AoE damage like the grenade launcher or space laser. I'd stay away from them.