Positioning/Team Fights/Anything as Necro
The standard necro builds put out a lot of damage but they are very glassy. You want to make sure you are making use of your long engagement range and try not to attract immediate attention in a group fight if possible.
Positioning is more important for necros than many other classes as it is one of the least mobile professions. Make use of advantageous terrain, such as going to the back side of keep on Forest or being on one of the platforms in foefire with objects to block line of sight.
Putting wells on top of someone in the downstate as their teammates go to rez = win. While it might be necessary to use your wells to help get someone into downstate initially, if you can save your wells until someone is downstate you can turn 1 enemy down into a team wipe.
Putrid mark is an amazingly strong ability and can be a game changer when used correctly against a heavy condi team. Using it effectively can turn a tough fight into an easy win. In fights between teams with 2 necros, he who putrid marks last, putrid marks best so try to be aware of their usage of it and then send those condis right back.
Your death shroud fear and fear mark can be used to save your kitten but if you can save them, they are more effectively used for downstate situations.