Substats: Crit Mutiplier&ATK&Fire, last one can be weak point or colossus, depending on ur usage.
Core: Orange core ATK, Green core Magazine. Free core can be crit mutiplier or fire ATK, more versatile or fire specialized.
Best use with hailey. 2 magazine core will grant u over 100 magazine and it's pretty enough for all hard solo colossus. If in the future tactical weapons can have fire rate cores, u can switch one of them to fire rate.
If u want fire specialization, u can put on fire core, and fire priority can be replaced to weak point mods. And if certain weak points of the enemy u r facing matches tactical weapons' weak point type, u can use overwhelm.
Since fire resistance of ordinary enemies are rather high so fire weapon may seen less effective in mobbing. But since 3 legions haven't cover 4 element type yet, missing one cold themed enemy type, and it will have low fire resistance. And this new legion might be released in upcoming updates, since elena and ult blair is on the way.