Keeping them alive is half preparation and half skilled gameplay. Some things you can do to prepare for a map is to run the right defensive auras that you need to if possible. For example on a chain map using grace since zombies have minimal armor or say palace using purity of lightning to help them facetank. Haste is a good aura for DPS but it is probably not as effective as using that slot to swap auras based on map mods, it really does help a ton. I don't know which slot you keep auras but you can find an item with a white socket such as bones of ullr for that pupose. Outside of auras, having 20/20 gems is one of the main ways they gain survivability. Most summoners use 1b3r for zombies in helm (RZ + Melee Splash + Multistrike + MPD) where you can swap one of the red gems for an Empower gem which will net them several thousand HP. You can also swap your weapon to Mon'tregul's Grasp which will cap their resistances if you have the 16% all resist zombie notable. It also consolidates zombies for tankier versions but you have less meat shield and consistency, a tradeoff that I find invaluable for dangerous maps. For increasing spectre survivability you can replace a blue gem with minion life if you need but I usually don't have trouble with them. You should use increased duration with evangelists if you aren't already and possible IAoE gem to help keep up bubble protection which is insanely valuable. Lastly, applying temp chains and enfeeble help and is good support for grouping. I typically just swap elemental weakness to enfeeble when I know i am about to face a hard map/boss/Exile from a box but if i had the ability to fit in temp chains I would recommend.

For gameplay, using at least 1 animation flask is advisable. It is not to help zombies but to help spectres, even if it seems like it doesn't heal much it really adds another level to them. I use one seething divine flask of animation and one ample (+20) hallowed life flask of animation. You have 3 uses of the instant and 5 of the heal over time per combat with no gained flask charges and it seems to work out well but its a rare flask to find, just look for a heal over time animation flask until you find one. Convocation is the major skill to help them avoid damage, but it has a cooldown so you cannot waste it. I use it mainly to keep spectres alive, not really zombies since they can be resummoned. Getting them off of elemental ground affects, dodging skills, and repositioning are all good uses. Keep in mind which damage skills get blocked by Evangelist bubbles and when it will be up and down, I commonly use convocation to position the mob right outside bubbles so my zombies melee from inside while the mobs skills are blocked. I also use Vaal Summon Skeletons and Summon Skeletons to act as shields. Animation Flasks due a wonderful job of keeping alive skeletons as they heal around half of their HP normally, combined with a possible bone offering to help meat shield while you spectres do work from ranged. If you happen to get the general with anger aura mobs will melt from the single target damage they take. You don't need to link anything with Skeletons for them to be useful. Vaal Haste is also really nice for clear speed/dps if you have the leech minion passives, as zombies have very high max HP they never leech cap so itll help them if they can connect with a mob. You can also use life leech with raise spectres which helps them almost always stay at max HP unless one shot but i don't find it neccessary. Using Rejuv Totem also helps as well as Vitality for having them survive AoE, especially combined with Mon'tregul's. Last thing is about directing them, using a move such as arc has seemed to be the best but after you convocation sometimes they run back, simply keep your auto attack on your bar and spam it or arc at a wall/ground and it will keep them from wandering back