Unable to deal damage to Alduin

The easiest way is Smithing and Enchanting. Barring that, simply level up. As you do so, Merchants inventories, quest rewards, and items found in the wild will improve.

Alternately, you could pursue some of Skyrims fabulous and exotic Artifacts, which are non-levelled magical items of exceptional power. Several of the Daedric ones are particularly good.

In the case of Armor, one important thing to note is that there is technically a 'cap' of 567 Armor - 80% physical damage reduction. A fully Smithing upgraded set of Hide armor yields somewhere in the neighborhood of 700 Armor with all the appropriate perks, so realistically, you can wear whatever looks good or has enchantments that you're fond of once you have the appropriate perks and upgrade it at a workbench.

While wielding a daedric dagger, having 100 one handed(all perks) and 100 sneak(all perks), the daedric dagger's base damage showed as 33. From the dagger's sneak attack(only 15x damage) it caused around 500 damage. Then I applied a power attack and it caused 1,000 damage. Then my orc used berzerk and a single strike caused 2,000 damage.

I could do some more testing (like getting shroud gloves, dual wielding with a power attack doesnt do a single hit unfortunately), and having bonus one handed damage enchants(it modifies the displayed base damage btw), but I think this is enough to show that multipliers stack (15x + 2x = 30x + 2x = 60x) instead of combine(15x + 2x = 16x + 2x = 17x).