Unsunghero has probably got the best advice. If you aren't recasting your pet until you get a max level you are going to have issues in general. Also a rusty halbred will max out your pets dmg at that low level so if you can prep and buy one before a long session that can help out.
Take a look here
https://wiki.project1999.com/Pet_Guide#Necromancer_Pets and just use the max dmg to tell if you have the max level pet. The best way to test is on something low level so you are more likely to get max dmg during the testing.
The other thing to help out at low levels is to have mend bones up. It has a bit of a recast, so cast it early then do some dmg and cast again. You can also use snare + fear as the others have suggested depending on the situation but engulfing darkness is a huge mana sink at lvl 12 so you might stick with clinging darkness. Another tactic is to cast your leach spell then go in tank a bit for your pet, back up lifetap to heal and go back in for a bit.
As you level up it gets easier and if you can find a duo with a druid or cleric or another pet class it makes life a lot easier. So basically play around with strategies a bit, even take on lower level stuff that is easier to get a feel for which strat you like best and then try the bandits again.
Nothing in this game is easy, some things are just easier. Necro is easier than a lot of other options but you still have to spend some time figuring out what works best for you.