1. When you join a world, do a scouting run. If the airport is occupied by a Top 5 (or heaven forbid, Rank 1 with valuation quadruple that of Rank 2), look around for a competition-free airport all around the world. Bankrupt, then restart at that airport.
2. In the event you do not restart, do not hold hard feelings when the mega-airline crushes your airline with capacity dumping.
3. If you move to another airport, the mega-airline may or may not show mercy by leaving you alone.
4. If he is merciful and leaves you alone, do not exact "revenge" on said mega-airline later in the game.
Especially since top airlines are usually Metaslaves. They know how to produce the DOP and quickly add/remove routes and may find ways of biting back, especially with their superior game knowledge.
Thank you all for your kind attention!