I comleted lvl 40 and must say I enjoyed it. 5/5. The only thing i dislike, when you are accidentaly teleported to starting position...
As for me one of the hardest lvl is lvl 36.
How on level 6? You must jump down along the arrow AFTER there are both portals on red signs in order to have enough energy. Another posibility is shot up,then down under the first portal, take energy then shoot on the right red sign on the floor and it shot you enough into air.
The Psionic is bit harder to play than the others two. Easies is juggernaut as for me. With juggernaut I won it without use any rune. The boss die on first try.
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 "Portal: The Flash Version"){kind=icon}
 "Talesworth Arena: Death Watch"){kind=icon}