Pheidippedes has some cool druid solo videos on youtube. This one shows him soloing the necrosis scarab in seb ng, root rotting and stepping out every summon.
https://www.youtube.com/watch?v=RUGH7qlnnH4 If you check the channel he has some impressive videos killing stronger targets in Chardok using a pet.
You're right the corner trick should work but he is just starting the cast as soon as he steps out of melee range -- summon is an ~11 second timer.