About 6 units that are the same faction as your monster plus any others units you might have to make it to the total of 15 should be enough. If you and your brother each bring about 5 buildings each, making a total of 10 on the map, which is enough to have fun and include some strategy. As for maps the one that comes with the Starter is pretty easy to play on.
The factions that are easy to play depend on the person but the Units for Terrasaurs can be said to each are good at one thing. Spikodon = Kills Unit clusters, Brontox = Holds Objectives/Makes more Units, Raptix = kills flying units, Carnidon = Brawl. This makes it easier to build a force around how you want to play.
This is not to say that you have to buy more products to have fun, but the new sets have filled in some holes and given new abilities to each faction making them all easier to play. This especially true for the Martian Menace.
Lords of Cthul in Set 1 Rise were really well rounded so they could also be considered easier to play.
Hope this helps and that you guys have fun.
HaphazardNinja
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Coolness is measured by collateral damage.
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