Playing on Ultra. Previously I was playing double waves and level 12 was as far as I got. Since you asked about the difficulty I did double waves until about wave 12 then did 1 by one and got to level 20 before I died to some dumb mistakes, Katana wielding Mini-Arker, 3 mortar turrets and 1 fireball
It plays as I'd expect, really hard but fair. Double waves are much more difficult than single ones the balls are the hardest enemy