Basically this. Top-tier Dragon Soul teams just vaporize everything and you net 34-40 souls per battle just from having Dragon Soul cast 2-3 times. Having other soul-generating troops (or Necromancy) isn’t necessary. If you get 30 souls (for simplicity) in 3 minutes vs 60 in 6 minutes, the former is getting you more battles won for the same souls. Not to assume you’re doing 6 minute battles. But if you speed up the battles, you speed up everything.
4 Dragons (can include duplicates) boosts Dragon Soul to 20 damage regardless of your level (and everything else). Krystenax is more powerful and has complementing colors, Sylvanimora is similar but with utility. Venbarak and Shadow Dragon have Magic Link.