Everything scales to your level at all times, with the exception of raid bosses I believe. But yes, I did this with krieg just using the firehawk and the hellborne tree, no rubi or orphanmaker.
The reason you use the orphanmaker is it's an extremely powerful shotgun that does damage to you. You use this to keep your shield down.
The rubi gives you 12% of damage you deal as health. Every bit of damage, including your firehawk. Grog nozzle is essentially the same, just with a 50% modifier rather than 12%. Doesn't do nearly as much damage, but much better chance to slag.
So with this and switching weapons consistently, you have a permanent flame of the firehawk (fire the orphanmaker consistently to keep your shields down) and obscene health gen (pull out grog nozzle when able to)