Your general strategy was correct, though this scenario posed a couple of interesting variants. For instance, you generally want to be very careful about putting your vehicles at any risk, since they usually are worth a lot of victory points. However in this particular scenario, I thought it was worth the risk to move the M-1 up close to the action for several reasons:
- I tried to prevent exposing the M-1 to enemy fire without first disrupting the enemy units that were within its LOS.
- My impression is that a vehicle like an M-1 with its extremely high armor protection will usually get immobolized by an enemy shot first as opposed to getting killed immediately. And if it became immobolized within the enemy's LOS, I felt I could still protect it sufficiently from that point forward by having it fire smoke rounds in front of it.
- After playing this scenario a number of times, I found the M-1's weapons to be surprisingly ineffective against the enemy's well-fortified positions. Therefore I felt I needed to have it fire at the enemy from a hex away so as to maximize it firepower. It is extremely rare that you will want to bring a tank into a hex that is adjacent to enemy units carrying RPGs, however I felt it was safe enough to do so, given the M-1's very strong armor protection coupled with my first two points above.
And although you said it was a mistake not to mass your Marines before the assault, it is generally very good practice not to stack your units whenever possible. However in this scenario you have to assault a strongly held position, so you will need to have all the units you can muster into a single assault force in order to dislodge the enemy. And you have the opportunity to stack them in a hex that is out of the enemy's LOS (if you fire your smoke correctly), so the enemy cannot take advantage of your units being stacked.
This is what you will have to strive for when performing any assaults - being able to outnumber the enemy during the assault while minimizing your units' exposure to enemy fire. In future games, you will often find yourself having to stack units for an assault while in the LOS of enemy units, so I thought this was a good scenario because it helped emphasize why you need to stack units for an assault, what the risk of doing it is, and how you can minimize your exposure (such as by using smoke and by moving in Ground Mode from two hexes away in for your assault). The scenario also emphasized the importance of softening up the enemy prior to an assault and it also made you think about which weapons could penetrate a heavily fortified position.