Just make the cast time 1 second (or 3/4) and add a recharge of 10 seconds. This would stop heroes being useless for 3 seconds (aftercast) and stop them from casting and casting and casting and casting.
Imo, as a human you should be playing with Order of Undeath + Ebon Vanguard Battle Standard of Honor as a human minion master. And therefore wont have much time to cast Death Nova anyhow
Im all for a change but I dont really see how that skill fits in to a human minionmaster bar, even now that its easier to keep control on minion health.
The 2 seconds are only annoying in PvE, since PvE has a faster pace in HM, and you'll need to spam more spells, since minions die faster when attacked by HM monsters.