They were created primarily for large slow movers like devolariums and sibleons, but they may work on others as well.
Tip 1 Using portal to stop shooting
If you area attacking a large npc and you can get it to start heading for the portal, leave it and run for the portal. After around 5 seconds go back and keep killing it. If you did it right the npc won't fire on you for a while.
Tip 2 Using portal to stop shooting
If you are dying and you can manage to "drag", "lead", or "bait" an npc to a portal and get other people to fire on it, you may then jump. If you jump RIGHT back, the kill will still be yours, but the npc will now be firing at whoever was shooting it when you left.