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On an overall build level, one is the "defensive" trait and the other is the "aggressive" trait, but neither trait really changes how the Assault plays. +5 Defense per enemy visible to a max of +20 OR +10% chance to crit per enemy visible to a max of +30%. It provides a solid foundation for the Assault's role of "the soldier who runs up to targets and blows them away, preferably flanking them." by letting them pull off stuff your other soldiers just can't do. That's 50% more move) When stacking other positive modifiers like the Skeleton Suit's +3 to move, it becomes even more favorable to Run & Gun, to boot. (So a Sprinter Support can use 16 move and shoot something, while a Run & Gun Assault can use 24 move and shoot something. A comparison can be made to the Support's Sprinter trait, and Run & Gun wins if you ignore that one is an activated ability with a cooldown while the other is automatic, as Sprinter is worth 4 move compared to a base per-action move of 12. The defining skill for the Assault, granting them vastly superior ability to flank targets than other classes. The Assault may fire or initiate Overwatch even after having used up all their actions in a turn.
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The Assault seems to lie somewhere between being your default line soldier and being a hyper-aggressive flanktastic murder machine that believes the best defense is a good offense.
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