Nothing. For a very simple reason: CipSoft can't and shouldn't be constantly buffing old content because it became too easy.
I understand the frustration of Ferumbras being ridiculously easy. But unfortunately that's part of the game evolution. How much harder was Gaz'Haragoth when CipSoft introduced it compared to now?
Over the years, for a given level, the game gets easier and easier. That's due to many factors, such as new equipment, new potions, imbuements, preys, charms, etc. This has always been the case with the game. If they buffed these bosses now, how long until they have to do it again? There's a reason why CipSoft stopped implementing these bosses, and the reason is that they don't scale very well (or at all). So instead they create bosses with a limited amount of players, for example, which limits the strength a team can have. They have also put a lot of effort in bosses that require more than brute force - take a look at The Nightmare Beast.
Buffing these old bosses would please the players for some time, but the problem would come back later. Instead they will have better results investing the manpower in new content.
Does it make sense that Ferumbras dies in 2 seconds and seems weaker than many other bosses? It doesn't. But a lot of things don't make sense in TIbia. For example, many new creatures are not as scary as a Demon, yet much stronger than them.