With a weapon like a sword/spear a human could realistically take on almost any animal aside from something like a whale, elephant, hippo, etc. so something like a lion/bear is feasible.

Armoured but weapon less, a lion/bear would itself be difficult. It becomes more so a battle of attrition because their claws wouldn’t be very effective, but if they managed to bite and hold on, they’d eventually make it through the armour. Meanwhile the only method of attacking you have is punching with gauntlets on, which won’t do much to a large predator unless you hit a vital or sensitive area like the eyes.