Take 1x Guardian Falcon to 4* 3.60 instead, the only 4* elemental defense debuffer.
Leveling plan
But once you have Falcon at 4* 3.60 ,
If you have purple 4* Tibertus at 4* 4.70 then Falcon to 4* 4.70 . Boldtusk, and Falcon, go well together because of very very high defense stats.
If no purple 4* Tibertus 4* 4.70 - and no red 4* Wilbur - then Gormek to 4* 4.70 but even with red 4* Wilbur, you will probably want to level Gormek eventually. Gormek, or Wilbur, gets you a generic / rainbow defense debuff red hero.
After - Gormek 4* 4.70 or Wilbur 4* 4.70 - and Falcon at 4* 4.70 then depends on rest of your roster - including new heroes you acquire - but likely Scarlett 4* 4.70 . Scarlett is a fast attack debuffer that protects your whole team ( I like team heroes better than solo heroes), and herself. But even dead Scarlett increases you red normal/ matching/ tile damage ( grave damage ). Scarlett, and similar hero yellow 4* Guardian Jackal, has a 5* HotM level of attack stat.
Also if you have yellow 4* Wu Kong, leveling Wu Kong - the only 4* mega attack buffer - should be a priority.