Nightsisters problem

Hello, devs and players alike. Need help. I run a squad with Rex lead, boba, dn, ep and sf. For a couple of days now, I've run by these mother talzin-lead nightsisters in my arena, specifically: MT lead, asajj, old daka, zombie and acolyte. Here's the thing. I easily killed old daka with boba's execute (which means she cant be revived permanently right?), then killed MT with my DN's annihilate (which means, again, she can't be revived permanently right?). But since acolyte and zombie has this neverending loop(duh), most of my squad got defeated leaving only boba and dn. Towards the end of the match, MT and Old daka are permanently revived. Here's the thing, I get the zombie's unrelenting revives (and the temporary revives of other NS) but why did old daka and MT got revived permanently? Does that mean boba and dn's execute/annihilate doesnt mean a thing against these NS? I'm just a f2p who invested a lot on dn and boba because of their unique skills. I know I really cant compete against p2p who has Nightsisters at gold gear already( although the squads that I encountered only had Talzin gold geared, I am currently ranked 43 right now) I just need an explanation. Any idea regarding this? Thank you.