there is either a timer or a set amount of puddles he will drop before he wipes you all
I counted something like 12mins last time I did it. there isn't that much room for mistakes if you want to do challange.
however it is a good challenge, the old form of the instance you just pulled up mobs super slow to make challenge very easy, sometimes taking over 30mins on just that boss fight alone. with good dps and good heals and tbh a guards force taughts are great in there too, you find the hardest part where to put the puddles to maximize the zombies you pull up. lots to think about for challenge mode but I won't spoil tactics cos I'm evil
if you want to just do T1 or not the challenge mode, the goal is too stick the puddles where you pull the least adds and kill the boss as quick as you can. under 10mins should be very possible if you focus 1 main dps on boss. remember that at half moral lodes of maggots will come in and suck your face if you stand still.