I have not tried it on hard and perhaps this helps the Rumanian forces a lot, but you can do a lot better than 2 rifle divisions in Odessa. With a little work and time, you can fortify the whole area around Odessa and it can pay off IMO.
The screen shot shown is turn 7 and the basic defense I plan on is in place. The Axis advance is slowed considerably by my screening force. If the 11th Army does not make a beeline for Odessa (or 1st PG doesn't come south to help), you should be able to get this into place. I gave the Coastal army a good commander and some good support units including some rail engineers and then basically dug in with a full army (24 points worth) of troops. Trying to dig Russians out of level 4 forts with 2 hex attacks can be time consuming and brutal. The Rumanians can't get any place against this type of Odessa defense. (In fact, I attacked them a couple of times with success). I eventually lost the hex NE of Odessa, but that was the one I could afford to lose. The AI tried several attacks, but my guys managed to hold. Eventually, the front moved on and I had Odessa isolated in the rear, but with a good sized garrison like this, I raised hell with the troops left in the area. Eventually, I brought in a cav unit to help make things more annoying. You don't want to know what happen during winter.
At any rate, there are several pluses to doing this. The first thing is that if the German does not react to it quickly, you are going to be well dug in and it will take most of 11th army several turns to clean it up. Even if your units get knocked out, they will simply route, even if you are cut off. This causes a big delay for the Germans trying to get 11th army to the Crimea, not to mention causing 11th army a good amount of losses, hopefully wearing them down where they are not quite as effective trying to get across the Dnepr and then they get to deal with trying to break into the Crimea. Should the Germans choose to ignore it, they really can't mask it off that well. You have a lot of troops and can do an active defense to make a German see the error of their ways. You also mess with rail line repairs in the area and they have to go further north with the rail repair unit when they may have had plans to go through Odessa.
This does use a fair number of troops, but I think the risks are worth it to have a really early big road bump the Germans really can't surround and they simply can't anchor their line in the south until Odessa is taken.