Hopoo feathers are one of, if not the best item for mithrix
Damage bonuses are multiplicative so a watch, glasses, ap rounds, focus crystal, crowbar will do way way way more damage then just stacking one of them
Stun grenades arnt that good, you shouldn't really stack them but this one is just mostly my own opinion
Mithrix will throw out spikes when he dashes so you don't wanna be moving in a straight direction while he does it, in general, NEVER move in a very straightforward way of moving, try to always move to the asides and slowly doing this big gradual circle
Leeching seed is pretty terrible and gets outclassed by harvester scythe 9/10 runs
Damage reduction like opals, tougher times, safer spaces, rose buckler are usually more valuable then healing, repulsion armor plates are an outliar because they reduce damage taken by 5 each so it's good for chip damage like damage of time statues effects, wisps, clay templar but will not do anything late game against most enemies, and this is also the reason why planula is the 2nd worst boss item in the game, boss items are so incredibly strong and this one is just 3 RAPs so like... Yea
You can (no longer, used to in the past) apply 2 bleed at once, so once you have shatterspleen and max crit, more tri tips won't do anything
Stacking atgs are good but you can achieve proc chains with other proc items, I won't get into it but atg - > ukelele - > sticky bomb is a TON of value
Please never pick up aegis it is so bad
I'm assuming you know how pizza works in general, but, the closer you are the slower it gets which makes it very easy to just slowly move around and keep shooting at mithrix, tons of free damage
It looks like it's your first time against phase 4, I won't really explain it but I will say all you need to do to achieve success. Start out the phase by hitting him while he's slauching and then... Start sprinting around him in big circles... If you sprint to his lefts and rights his spikes are unable to hit you