Medusa: Arena build

Warrior Tabi: I would agree with Bran that, although Warrior has a bit better early game than Ninja, it would arguably be more beneficial to move into the final build as quickly as possible in a game mode such as arena. The fact that you include Qin's Sais reinforces this premise.

The Crusher: This feels a bit weak to me in the second position. The low power and attack speed usually are not preferred over getting some raw power online first for better DPS. The low power and attack speed is especially underwhelming if you do decide to start with Ninja Tabi instead of Warrior.

Transcendence: I'm okay with this, although I personally don't go for stacking items in game modes like this myself. As a possible alternative, an item that I have become fond of running on my hunters in arena is Shifter's Shield. It gives you some nice raw power upon completion without needing to be stacked over time, as well as provides you with some appreciated survivability (solid anti-physical and decent magical protection) as you attempt to return to your fountain to heal up.

Alternatively, you could start with something like Bloodforge as well if you do want the lifesteal and think you will be getting kills often to utilize the passive.

Qin's Sais & The Executioner: Solid item combo.

Brawler's Beat Stick: I feel that this is more of a situational item. Not bad generally speaking though.

Other Thoughts: This section is more opinionated, so should be taken with a grain of salt, but I personally feel that there are certain items that simply work better in teamfight game modes such as arena.

Odysseus' Bow: In a build focusing on high power such as this one, the passive from Ody Bow is very effective in arena where enemies are usually grouped together. This provides you with solid poke damage as well as can help your DPS in something like a Qin's build.

Rage: I'm a big fan of building crit in arena with hunters, and this item is a big reason why. Normally, you can stack this item's passive quickly/easily, and allows you to focus on burst damage instead of constant, gradual damage that wears down teams. If you have a team with high crowd control with good setup potential, burst damage from crits can be easy and effective to build into.

Hunter's Blessing: Although you do have the option to bypass a Blessing and rush into boots, this Blessing (especially its evolved form) is too good for me to pass up for only 800 gold.

With Qin's as your main DPS item, you're best served going with Ninja Tabi IMO.

Also...Trans in this build is a bit weird...? It's not bad cuz high power, just...weird.