Stop worrying about reinforcements. Start the battle by putting three elemental orbs on the boss, switch to Über-Mythra halfway through your fourth combo and do a fifth combo that you can freely select thanks to Über-Mythra's ability. If you manage to pull off five orbs and instantly activate a chain attack, you can go to town with Über-Mythra in each round and should be able to deplete at least half of the boss's health bar at which point he's dead for sure.

A full elemental break can be done with only four elemental bursts, so you may shorten the above by one cycle and still come out victorious. Point is that with such a strategy the boss doesn't even get to summon reinforcements.

Über-Mythra attacks all elemental orbs at once. For your other party members, attacking a weakness guarantees a double hit to that element, so it's a given that you can reach round 2 of the chain attack. In round 2 Über-Mythra reduces all remaining orbs to 1 hit point, so you are free to do whatever with your other party members as anything will lead to an elemental burst and assures you round 3. 

I can't get 4 orbs in phase 2 before he summons robots and they murder me. I tried with three but the final boss is stupidly tanky and I can't even get 1/4 of his health down with 3 orbs.

You also didn't address that the seal reinforcements mechanic is blatantly broken.

Which makes me wonder how messed up your party is, because the robots he summons aren't particularly powerful. As usual when fighting a group of enemies, Tora as tank does wonders when he draws the attention of enemies. Nia as healer completes the standard combo that works well, especially if you have Dromarch, Vess and Nim equipped on her. The advantage of a blade like Nim is that you are free to use chain attacks in situations where you fear that someone of your party is going to die soon, because using her level 1 special during a chain attack refills everyone's health 100%, so you don't need to save the group gauge for reviving.

Mythra is incredibly fast at completing a combo, even all on your own. Once you've filled out her affinity chart, she commonly gets recharges of the XYB attack you use for an immediate additional use and that's how you build up specials fast. You can go either light/light/light with her alone or light/light/water with Dromarch's support. Taking down a robot shouldn't take that long.

Have you equipped your characters with accessories that boost stats like attack and agility by X%? Have you equipped Tora with accessories that help him to build more aggro? The accessory he comes with in the beginning of the game boosts aggro with each passing second. Also, check his XYB abilities and make sure to add the one that blocks and replenishes HP by 5-10% upon every hit he takes. Tora is a beastly tank and can absorb so many hits that Rex and Nia will be pretty safe.

On your blades you can equip similar accessories that increase your damage output, or in Poppi's case, damage reduction. Poppi can be boosted in a major way by playing Tiger! Tiger! and earning parts. In the menu where you can buy parts for Poppi, you can press Y to sell what you don't need, so gathering ether isn't that grindy. All blades can be enhanced with crystals to improve their auto attacks and at times get special perks for their weapon.

If you are level 72, you shouldn't have much problems to complete the game. But if you do have problems, you should go over your equipment and look for improvements. Unlike in the previous Xenoblade games, the number of equipment has been greatly reduced, so it's not going to take long to fix it. What takes some time is filling out affinity charts of blades, but doing four of the five rings should be sufficient. For Mythra, doing all five provides her great benefits that I mentioned above.

I got Ursula on my second file early on (15 hours of playtime; didn't get her on my first playthrough) and completed her sidequest in 35 hours. The sad thing is that this isn't the worst thing about the game because she is bearable if she is among the first rare blades you draw.

The worst thing is that the draw rate for rare blades becomes unbearable once you miss only five of them. You can spend dozens of core crystals without getting anything while the game trolls you with tips like "the more you resonate, the better your chances at a rare blade." In reality, the more rare blades you have, the lower your chances get.

I am missing three rare blades at this point and it's a damn lottery. It messes up completion so hard because it postpones building trust with blades. I do not even know if I'll be lucky enough to get all of the missing blades. Either they patch this and make it reasonable or they implement a new game+ that allows you to carry over all unlocked blades including their affinity charts while resetting the odds of drawing rare blades.

With that logic Ursula isn't a problem either. You have 15 active blades and there are 38 rare blades; forego the use of any blade you don't like.