I have only used them when i get to assault in planetstrike, three five man units each with a melta gun and power fist to knock out bastions or tanks. they also force the other player to split their fire between them and my LRC and rhinos
Black Templars don't usually need Assault Squads since the entire army is capable of moving so fast due to Righteous Zeal. The best use for Assault Marines is to give them meltabombs since each squad member is able to take them then go expensive vehicle hunting. A small unit will oftentimes manage to get into combat with the right kind of target and it usually only takes one or two hits with such powerful weapons.
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