Where to kill 11+ enemies in a row to get 1UP (without glitch/turtle trick)?

From Super Mario Bros./Secrets Wiki

1. Multiple Kills When enemies approach you in great numbers, get them in a row to score high points. Points will double (100, 200, 400) for each enemy you get. Goombas are the best foes on which to test this technique.
2. Double Kills When you stomp on two enemies at a time, your score increases accordingly (100 + 400).
3. Kick and run When you use the stomped turtle to defeat the other enemies, your score will increase with each additional kill (500, 800, 1000, 2000, 4000, 5000, 8000, 1-up). If you do this enough times, you'll get an extra life!
4. Other secret tactics There is a tactic called "wait and re-stomp" where you can wait for the turtle to revive after you stomp it, and then stomp it again for a high score. This "double stomp" tactic will change the 100 + 400 score into a 100 + 500 score.
5. Other variations of this tactic can also increase your score. One example is the "triple kill and kick." This high-level tactic will give you 100 + 200 + 400 + 1000 points. Depending on your skill, you can combine your attacks to increase your score. If you're going to do it, do it in style!

From what I gather, there may exist locations where you can piece together combos in order to reach the 11, or more, kills in order to achieve a 1UP. But why would you want to if you can just get 128 (the max) lives on World 1-1?

The 4-1 example you gave looks like a place where this is possible. Use the green shell to box in the spinies, wait for Lakitu to generate 11+ spinies, then use the green shell to kill them all at once.

World 8-2 is a better place for this, since it generates green turtles naturally, rather than requiring a glitch to turn a spiny into a green turtle.