Well, 3CP is 1CP after Guards and 45mm; and AT Grenades will be around too
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Also I want to try vetting this vehicles with no guns up just to see if that is possible.
It will only be doable by running people over or driving through fires
You can't run people over with the halftrack. Light vehicles don't have the right crush stats.
Also, you are right about the guards and the 45mm AT gun. I disregarded them for 3 reasons:
1) I see a shock trooper response 90% of the time when I go for assault grenadiers.
2) The mech assault group is a substitute for the flamer halftrack, which you probably would not be getting anyway if you see early guards.
However, even when there are guards or 45mm guns, the mech assault group is still excellent for clearing out flanks.