How could one Dalek hold against entire armada of Daleks?

Question

In recent Christmas special episode of Doctor Who Twice Upon A Time,

The Doctor sought help from a good Dalek. We learned in the episode that normal Daleks wanted to destroy this anomaly. They sent armies upon armies for thousands of years, but failed to kill one Dalek.

How did that one Dalek achieve a feat which the entire Time Lord race had had a hard time with?

Let's say, that good Dalek could anticipate every moves of his enemies as he had access to Dalek hive mind, the reverse is also true. Others could anticipate his moves, too. So, I don't think this is the case.

Answer 1

Short answer is : We don't know

Rusty was first introduced in season 8, the day he became obsessed with destroying Daleks. In this episode we see him destroy several Daleks (the exact number is hard to tell, but judging by the carcasses, more than 5) using the element of surprise. He then returns to the Daleks in order to destroy them, hinting to he was destroying them from the inside for a while. As far as I know, no more record of him was shown in the series until 2017 Christmas special.

Speculation time

We had a glance at how the Daleks react to abnormality among them in the first episode of season 7, The Dalek Asylum. In this we learn that the Daleks are reluctant to extinguish hatred, as it is a thing of beauty to them. So they might have "just" restrain Rusty's movement, but let his hatred go on.

We also learn in this episode that the Daleks are afraid of their insane brethren, and would rather go and capture the Doctor, not kill him, and send him in their place. We can assume this excuse of "too scared to go" applies also to the case of Rusty. Not a very good point in my opinion, but we have seen worse in the Who-verse

Answer 2

This Dalek has looked into the mind of the doctor I'm sure a casual look into his/her wobbly wobbly timely wimey war insanity would have given the dalek examples of the terror of the daleks due to the doctor fighting in the time war, and who knows what the doctor saw the time lords doing to the daleks and vice versa that may have helped Rusty "Think" of "tips" to hold back an armada. Like for example in "Dr Who Doomsday" a tenth doctor episode, the time lords used a "Dalek Prison" called "The Genesis Ark" which imprisoned thousands of Daleks if not more and who knows the "other weapons" the time lords used and like in the first female doctor episode "the woman who fell to earth" The Doctor are quite ingenious in building stuff.