He said: 'if the shield isn't removed' because in that case is very likely that mabui gets hit by a faster first prompt move enemy ninja and that so she heals herself and not tenten. Obviously garv meant in the case the enemy is faster. If is slower and tenten have lower hp than other ninjas you are right, but in that case is even right that you lose, because you should have used the acupuncture to another ninja, for example mabui after azure fang healed, so that you could have used a mystery on her in round2.

Who was your first prompt move? If was roshi, indeed, if was yugito nii there's no way. Yugito standard 75% of the times is 10 combo and even if she dies is not a problem since resurrects on round 3. If you had higher initiative you would have dealt before mabui moved both ignition and poison to the whole team, then mabui moved and the enemy used tenten. At that point you copy mei. If you are lucky your roshi heals himself and starts a combo that acupuncture mabui. If the enemy is lucky tenten starts a combo on yugito that is not said that dies without the boost of the barrier and even if dies is not a problem. Lets consider the case she dies. Your yagura moves and damages again mabui and apply another stack of poison. Enemy mei hit your roshi and gives him 1 more stack of buff. Your azure fang heals roshi almost to max hp, enemy af heals mabui of debuffs, enemy clone hits your mei clone, mei clone hits enemy clone and applies poison to two characters. Ignition and poison damage apply to who is targeted.

Round 2: Lets say the enemy is not * and hits your main with mabui to start a combo that immobilizes it. You wait and use roshi on his azure fang as soon as he uses the healing mystery so you acupuncture it. Yes, mei will use his mystery but with no barrier the damage is low and you only suffer ignition. Most likely you are at half of your hp the same the enemy due to poison and ignition.

Round 3: yugito resurrects and asap you use her mystery on tenten so yagura acupuncture her and this time you copy mabui, so you get shields for your main and yagura and also a cleaner if needed and you heal with your mystery. Before mabui moves yugito hits again with her 10 combo standard and if you are lucky you deliver again all the combo effects. Unless you are very depowered somebody at this point dies, very likely tenten since usually is the lowest hp one on purpose and you have won.

I've used that team for a long time and still is one of my expendable teams in matsuri. But yes, there are many ways to counter that team now.

Mei mabui tenten team heavily depends on chakra. Ninjas like nagato, bee samehada and kisame are counters against it if you own initiative. Other counters are ninjas able to stop mabui in round 1 (theoretically all the mains are able to counter it by themselves currently. Mb, sb and cf can directly do it even without initiative, af and cf without 1st mystery but with immobile passive can do it owning initiative and mb can acupuncture tenten after mabui healed).