gaara is in a dessert here,tsunade's SOH expired after a few hours,gaara has had his sand armour for 2 days,his chakra in the 2nd data book was 5 while tsunade in the 3rd databook was 4,she isnt outlasting gaara.
OP says gaara already has his sensor sand,so mist is useless as he will spot them,tsunade's healing wont really help if he hits a sand burial would it?,because it leaves behind nothing but a bloody pile,sand hail is enough to capture both of them.
Water dragon is useless,gaara already hid his sand inside it and caught madara,he is in a dessert as well,gaara can fly,then their attacks will be futile,he rose tons of sand from the ground to where he was high in the sky in one panel with a wave of the hand.
I am not underestimating them,i am stating the obvious here,no way they can win in a dessert. Mei wont last long,tsunade wont do much against a flying gaara.
Your points are invalid.
Firstly Tsunade's Endurance Levels versus Gaara. When Tsunade got to the battle field she restored Gaara's chakra and his health. She did the same with Oonoki. And Mei. And Oonoki again. Which means 4 of the kages started of at 100% but Tsunade's charka reserves went into healing the others first.
The databook has incongruent statements and featwise Tsunade has outlasted Gaara. Especially considering how Tsunade herself said healing requires a lot of chakra.
You underestimate the usages of mist. When Mei trapped Sasuke she used an acid mist technique which she absorbed as to not to harm the other kages. And logistically why she doesn't use it now. In this fight though Tsunade can heal from the damage done by the mist with Byakougou. Gaara though has not such defense against the acid mist. In effect it will either distract him, lower his resolve, lead to his eventual defeat as he'd eventually melt away or all of the above. Given how large an area Mei manage to cover with the hiding in the mist jutsu when fighting Madara she can create an expansive fog that Gaara would have trouble getting out of.
Water Dragon is not useless. The Water Dragon helped the sand get to Madara. But in this case to two jutsus would clash and given the immensity of her her water dragons, and the way it tossed Susanoo around I'm willing to bet that if the two jutsus came head to head The Water would bulldoze through the sand. It will make the sand heavier (that's kind of how sand castles are made) we know that heavier sand is harder for Gaara to manipulate which is why Oonoki lightened Gaara's Sand. Yes Gaara can still manipulate drenched sand but not to the degree to which he can manipulate dry sand.
Yes Gaara can fly but Tsunade is like a giant Energizer bunny, meaning she can power all of Mei's jutsus to entirely new levels the same we she powered up Oonoki's Jinton. A much larger water dragon jutsu for instance will be difficult for Gaara to avoid.
Not to mention Katsuyu. These two kunoichis cause enough problems. But there is nothing Gaara can do to Katsuyu whereas if he is caught of guard acid when melt him away instantly.
Gaara makes large waves of Sand I will grant you that but consider this.Tsunade is the world's best medical ninja and we know that based on what she taught Sakura the healing is the secondary function of a medical nin, and avoiding attacks is the the primary task of a medical nin because if they die there's no one to heal the team. Therefore it only stands to reason that Tsunade is one of the most evasive characters in Naruto if her primary skill is evasion when her secondary skill is unparalleled. People mistake her decision to tank as an inability to avoid. But Madara's Katon is a perfect example of how she could have done either but chose to tank the attack. My point being I don't think techniques like Sand Hail will pose much trouble for her, and based on her jumping feats, it's quite possible she can jump well over Sand Tsunami or jump out of it's range. It's also possible that Tsunade can attack Gaara while he is floating on his sand because of how high she can jump.
Do you know what turns sand to glass? Heat and pressure. Mei can try to crystalize the sand with lava and tsunade's strength. As far as what's been shown Gaara cannot manipulate Glass. Which means the can render more and more of Gaara's weapon useless and even use it against him the same way Tsunade used Madara's Susanoo swords against him.
I see people are denying Mei's prowess with doton and katon but logically if Her Kgs which use those elements are powerful it only serves to reason that the elements would be powerful when used singularly. And technically Mei should be able to first heat the sand with a Katon and then apply pressure with Doton to make Gaara's weapon unusable on a much large scale.
In addition we have seen Tsunade turn her chakra into electricity there is no reason she can't just use a higher volume of chakra and do the same thing as A during the group gokage attack.