@Tamer-of-Beasts I have found that as long as the Central Powers reach Bombay by rounds 5 to 8 then they have a good chance of succeeding. The total troops the CPs can reach Bombay by with by that time is well over 100, against a British force of 60 to 90 troops. While the CPs are at a disadvantage because they are attacking as three armies individually and will suffer more casualties, they should have an overwhelming ratio of troops compared to the British.
(Note: the CPs could hypothetically reach Bombay by round 8 with over 150 units, this comes at the cost of losing many of the territories in the west.)
In the game that I won using this strategy, the British had produced and stationed in Bombay an army of about 85 units with air-superiority. Meanwhile, the CPs had a combined force of about 120 units (including reinforcements that arrived in subsequent rounds of the battle in Bombay). While I might have had a little bit of luck on the dice rolls it certainly was evened out by the lack of air-superiority. The Russians are the only roadblock for the CPs, but they are forced to abandon defending Moscow if they are to have a significant effect. If they send half of their army to Moscow and to block the CPs, then the CPs will destroy both armies individually or destroy the blocking army and trigger the Russian Revolution. If the Russians send their entire army to block the CPs, then the CPs will simply defeat Russia and move back to the western front with practically no casualties. Most players who control Russia will never even consider abandoning Moscow, and the defence of their country, just to block a “possible” attack against Bombay. Therefore, while the strategy is not perfect due to the Russians ability to block the CPs, the British can do little to defend Bombay by themselves.