For you to not have this problem, try to get as much minion feeded from early. at 18 min mark you should be 150 minions. If you succeed with this you should not have any problems getting kills (except if your enemies are too feeded).
TIP: Practice farm on custom games. There you should even get almost 200 minions on 18 min mark.

No matter if your team is from unskilled players. Stay at the back and engage only after your team. Kill the easy low healthed targets 1st so if all your team members die you wont have much troubles killing and finishing your enemies. Still, try to keep most of your team alive. Spetially the tank.

This build is good not only because for the huge health but for the big amount of damage your gonna deal. As most of you know, here Atma's Empaler does not show the damage bonus so imagine how much bonus damage you will have. The bloodthirster grants you with enough life steal to keep you up in a team fight or if your low to heal fast enough not to lose any XP or miss a fight. The amount of health you have makes you very tough target on the battlefield but in the same time very dangerous and deadly killer. The attack speed + passive + the damage you have = unhappy enemies and win for your team.