From Brotato Wiki
Damage modifications are increased by 100%
You can only equip one weapon at a time
How to Unlock
Win a run in Danger 0
Unlocks Focus if you win a game with One Armed:
One Armed has the second broadest range of starting weapons in the game. They are also one of the few characters who can start with an Ethereal weapon.
The broad range of weapons allows for scaling off any base stat, except for Armor.
Guide and Strategy
One Handed has triple regular Attack Speed (+200%), and Damage modifications are doubled (+100%). This makes hard-hitting weapons especially attractive (due to the high attack rate), and it means that all damage modifications are very economical.
However, One Armed can only hold one weapon, and this makes fighting multiple targets much harder than with most other characters. Therefore, weapons that have a cleave (piercing) or bounce effect are very desirable on One Armed. Bounce effects in particular reach across the entire map, because bouncing projectiles have unlimited range.
Lightning Shiv is the strongest option.
Its damage scales well with Melee Damage and Elemental Damage, both of which are easy to find on good items. Elemental damage is more valuable on this weapon than melee damage, so it should be prioritized; if you aim to kill Elites, this requires melee damage, because the elite needs to be focused directly, which mainly causes melee damage.
Like with all melee weapons, the melee part of the attack will cleave, i.e. hit multiple enemies in its path. This becomes easier and more effective with more Range. Cleaving will create additional lightning projectiles (one for each enemy hit), which can be a tremendous DPS increase. Always try to cleave with your attacks. Extra Range will also make killing Elites easier, or even possible at all (depending on the Elite).
Lightning projectiles can Pierce, if you find the items that enable it. This is usually a DPS increase, and also increases secondary effects.
Slingshot is a good ranged option.
It only scales off Ranged Damage, which is difficult to find in the shop, so you need to rely on level upgrades more (and favourable RNG). Slingshot projectiles can inflict burn with Scared Sausage; this opens a secondary damage path and helps to deal with groups of enemies and hordes. If you want to go that route, also get Snake and, of course, some Elemental Damage.
Your single target damage can be very high with this weapon, which makes it good to kill elites. However, since your projectiles do not pierce unless you have items that enable this, it is weaker when dealing with groups of enemies than other weapons.
Getting piercing from items is very desirable with a Slingshot.
Damage stats are highly efficient, because their value is doubled. Attack Speed is still useful, although you have a lot of it already, but it does not have the double value modifier. This makes it relatively more expensive compared to damage.
If you use a weapon that scales with Ranged Damage, then look for that stat especially during level ups (upgrades), because the stat is relatively rare in the shop.
With the weapons that are strongest on One Armed, Life Steal is quite effective (all projectiles can trigger the effect on each hit, multiple times if piercing). This, and your high attack speed, will create a lot of opportunities for life stealing to happen.
If you use a weapon that does not bounce (usually a gun), Range becomes an important stat for you, because you cannot get the usual set bonus of +50 Range for 6 guns. High range will increase your damage uptime and therefore DPS, and makes it easier to stay safely away from danger. Piercing bullets will reach further (possibly hitting one more enemy that they would have with less range). Take Range if you see a good opportunity.
Prioritize Ranged Damage, if you can use that, then Armor (both are quite rare in the shop).
Upgrades should always be opportunistic, depending on what the options are.
Do not tunnel vision on damage.
All items that provide Damage benefits are much more valuable for you than for other characters, because of your trait. This goes both ways, so the opposite is true for negative Damage. Items should be chosen accordingly.
However, do not blindly pick anything with Damage on it, because many items also have a downside, usually some negative stat. Since there is enough Damage to be found, you can pick and choose, and do not have to dump valuable side stats (such as Speed and HP). Many of the early %Damage items have negative defensive stats on them, which will make you more vulnerable, while not increasing your effective damage by that much (yet).
%Damage becomes better with higher base damage stats; it is therefore not that noticeable early on, and will only pull its weight if you also get the base damage for your build. This is very important to keep in mind when building One Armed (and other characters as well).
Upgrading the weapon
It is obviously important to upgrade your only weapon to the highest tier (IV) as soon as possible. High Luck can help with finding the later upgrades a bit sooner. Because it is not possible to combine multiple layers of weapon tiers, since you can only hold the one weapon, you need to find the higher tiers in the shop. It's OK to go from T2 to T3 by recycling and replacing, in case you find an early T3; then wait for another T3 to turn up in order to get to T4. You can also lock the T3 and wait for another T2, to go straight to T4, but this will block a shop slot, which is an opportunity cost.
In order to not waste materials on rerolls, keep in mind that the first 5 shops have special restrictions for what they will display: the first two shops will always display 2 weapons, the third shop will always display 1 weapon, and the fourth and fifth shop will display at least 1 weapon.
Also observe that, assuming you want to keep your starting weapon, you will need every tier level of your weapon exactly once, in order to get to T4. So when you have your T2, you are only interested in T2+, then only T3+. Ideally you want to upgrade and not recycle.
Since the first two shops only offer T1 weapons, you can not upgrade past T2. If you then have 2 items locked in these shops, rerolling becomes a waste of materials.
In the third shop, the chance for a T2 weapon is only 12%, and it is not guaranteed to be your weapon. In this shop, there is only 1 weapon on display, so rerolling is not economical in shop 3, especially not if you also have items locked.
Finally, to increase the chances for more items on rerolls in shops 4 and 5, you can lock a weapon (ideally the one that you need) before you reroll.
Secondary and utility items
Items that cause damage ("secondary damage") such as Cyberball, Alien Eyes and Baby with a Beard are strong on One Armed, because you typically have high %Damage. These items are only good, however, if you also hold their respective base damage stat (depends on the item).
Silver Bullet in particular can be a very powerful item on One Armed: its value is doubled (by your trait), and its effect is multiplicative with your existing %Damage. It will very often turn elites into cannon fodder if your %Damage is high.
Engineering items and the Engineering stat are rather weak, because the items do not profit from your Damage stat, and the stat itself does not have the +100% modifier. Engineering related items should therefore have low priority, but can still be a decent source of secondary damage. Turrets and Landmines can spread burn with Scared Sausage. The item Improved Tools applies half of your Attack Speed bonus to all Engineering structures; this is very strong. If you get this item, all Engineering items become very good value.
The value of healing and utility items depends on your skill and playstyle, and choice of main weapon (if you attack from range, you require fewer defenses).
Economic items and harvesting should be valued highly, as with mostly any character.