I guess you’re right about that, because Proteus is pretty darn broken for a 4* hero lol 
But in all seriousness, his special’s DoT isn’t the main attraction here. It’s blocking Mana. If you use two Proteus’ then just fire the first one, and don’t fire the 2nd one until the first mana block runs out. Like PirateKing said that’ll make it so the bosses never fire, makes your class trail way way easier.
Also in case you are ever unsure of what status effect overwrites what, I made a topic that lists that. As you can see, Poison will overwrite Poison: 📑 Status Effects (Buffs and Ailments) and Stacks - Dead topic see link in post #190