Ah we add buffs into equation now? Buff the bear with everything you want, he still won't compare to heroes / trained troops / summons by the time you pick growth / shrink in life mastery.
Now imagine I cast that growth on any tarth unit attacking the bear. My unit has 10 atk for sake of simple calculation. So he sits at 15 atk now. Then we add 50% trait = 15+7.5 = 22 if you round down, 23 if you round up. Now add double strike. And I can pimp that units with upgraded armor and weapons, give it a mount so it will always go before your bear (can't care less if your bear is defending, 0 defense when defending becomes 5 defense), as well as pimp it with accessories (necklaces, rings etc).
Actually, the real power of beastlord seems to be the ability to capture a Great Wolf. It buffs all friendly units with + its level to attack for the duration of combat. Not only does this make those bears more dangerous (+<wolf-level> to each attack of maul, and it can stack with spells like growth), but it also works on trained units. So each figure of each unit gets an attack buff; if you have 4 spearmen, the unit basically gets 4 * <wolf-level> to attack. The wolf itself is fairly fragile, but if you can keep it alive while you level it up, it's arguably the most powerful unit in the game. E.g. with endgame tech, if a level 20 great wolf is in an army with 8 other units, each of which has 6 figures in it, the wolf can effectively buff the total offensive strength of the army by 6*8*20=960.
Of course, this is balanced by the fragility of the unit; it's nearly impossible to get one to that high a level without save-scumming whenever it gets killed by magic/ranged units (and this will be even worse with the new shills).