If you’re trying to convey the idea that this would happen regularly, you could use the following construct:

avait l’habitude de + infinitif

which would translate literally as “had the habit” but can be used to mean “used to”

Your example would become

Son frère, allant à la vitesse de l’éclair, avait l’habitude de se glisser derrière lui et de l’endormir l’aide d’une prise de lutte.

I would use the imparfait here, but accompanied by a word or phrase to express how often the event happened. Not using anything would be for something really daily, not occasional like your example. I think you wouldn’t use “would” there.

So you can say things like :

De temps à autres, son frère, allant à la vitesse de l’éclair, se glissait derrière lui et l’endormissait à l’aide d’une prise de lutte

You can use : de temps en temps, parfois, des fois, …. They are used for various periods of time.

As Nico Mezet said, you can also use avoir l’habitude de, which then changes the tense to infinitive. Notice that “avoir” is in imparfait.

To me, it seems like the easiest and most natural way to say it.

I also read in the comments that it’s meant for casual speech, I didn’t correct anything yet because it’s not the topic of your question, but that is not at all how you would say it casually.

I said “De temps à autres” in my example, but the other three are way better fit for casual talk. Other words, like “se glisser” and “lutte” are not very casual-souding either, but the most important thing that you need to remember is that participe présent (“allant”) is noooooot casual. At all. It’s almost never spoken, actually, it’s something you read.

It really doesn’t have the same level of formality as the “verb+ing” in english. There is quite the different though between “en allant” and “allant“, there are not used in same contexts, but “en allant” can be causal as well. Tell me if you want more but again, it’s off-topic and my post is already long 😉