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I have found Vision IAS is the best one for mains.
Enroll in its 20 or 25 full length mains exam test series. Here they will cover subject wise twice before going for full length.
Do not just give full length one that 8 test wala Series instead enroll for 20–25 tests series. This is subject wise and then full length.
Advantages:
Though it has some drawbacks:
Hacks:
There is an alternatives like Forum IAS mains guidance programme. It's mentoring system is good. You can ask your friends.
Whomever you find better you can go, enroll and start writing mains mock exam. Where ever you may enroll start giving mock exam honestly and seriously.