The best replacement for flux core is mig.

The best alternatives to flux core are rated as following:
★★★★★  mig
★★  mig welding
★★  stick welder
★  stick welding
★  metal core
★  shielding gas
★  gas welding
★  solid wire
★  gas mig welding
★  hardwire
  gas mig
  dual shield welding
  stick
  rosin core
  mig welder
  arc welder

Your alternative for flux core is not listed? Suggest!
Speaking about flux core, one probably shouldn't forget about shielding gas.

Other search terms related to flux core and their ratings are:
★★★★★ shielding gas
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flux core
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