This can be done on the Gold Fury or Fire Giant in Conquest or the Siege Monster in Siege. To accomplish this you must steal the kill after the enemy has done 51% of the damage.
To accomplish this you will need to know when the enemy is doing one of the objectives and at what health the objective is. The best way to do this is with wards (either normal or sentry). This will give you visibility without putting you in danger.
Next you have to be able to get in to the middle of the group and either deal a lot of damage to finish the kill with an ability or use Hand of the God. If you have Hand of the God 3 then you can use this to deal 1000 true damage which can great assist you with the steal.
A good way to get into the fight without getting CC'd or killed is to use blink through a wall so they don't see you coming. Guardians are great for this since they are naturally tanky and many can CC the enemy in an area with their Ult (Geb and Sylvanus are great for this).