If you're going Ranged, Riposte is useless to you. So is Lunge. Bigger Guns is also something of a waste. As a Mercenary, you're incentivized to raise Physique, even if you're going Ranged.
The real questions are:
Carpet Grenades or Detonation Charge? I played a Melee Merc, but I never actually chose one.
Rending Attacks or Giant Slayer? I took Rending Attacks, but you'd be surprised how often Giant Slayer applies. Granted, Sunder may be more useful (when it applies) than Stagger but Ranged already ignores some Armor. If you really want Sunder, you can pick up a
Shoulder Grunch.
Equipment
Tough applies to Shields, as well.
Concentrate Fire means you don't want to miss. Bonus Hit Rate really helps with that. Most Bonus Hit Rate Ranged weapons also have Can't Crit.
You could also build for Crits, because Mercenaries get +10 Crit chance. With all the bonuses, I'm fairly certain you could have 25%/30% (without Giant Slayer).*
Don't be afraid to use Energy Weapons. The +20% only applies to base weapon damage. You get Aim/2 as bonus damage.
I found you still don't want to neglect Shields. Maybe it's just because Tech Specialists are absurdly tanky, but I found my Melee Merc a lot squishier than I was expecting. That said, I'm fairly sure Ranged clears encounters faster.
Don't let your total Evasion dip negative. You take extra damage. If you want to wear the Ramshackle Power Armor, also wear the Light Jetpack or use the Rouser.
*Ranged attacks are bugged right now (0.9.002), so they can't Crit. Smuggler's Aimed Shot and Alpha Strike don't work either. Hopefully, it gets fixed soon.
Transformatives
Most Transformatives don't have a combat benefit. This is by design, as in the previous game (Corruption of Champions), many players felt forced to choose 'optimal' transformations, rather than the ones that appealed to them aesthetically. There are still some that slipped through, though.
There is a '
Show me your characters' thread, if you want an idea of the kinds of characters people are creating.