Munitions Training 6a or 6b with shotguns

For the Krogan Merc, Sustained Fire increases melee damage significantly, especially if you also have Melee Synergy. Shoot shotgun, then headbutt... If you have Martial Arts also working, everything dies quickly. It's a great melee build, if you like headbutts, and I do....

RAWWRR!
My current melee Krogan Merc has only 1 in Flamethrower (it's still useful). Power Armor (4B) in Fortify adds another 30% melee damage. I use Unmoving Defense to take a break and recover shields. For Munitions Training I use 4B (+20% melee dmg), 5A (Melee Synergy) and 6A (Sustained Fire). I use Rage 4A, 5B (Martial Arts can give +65% melee dmg), and 6A. I also use Disruptor Ammo (which stuns), and the Heavy Melee mod (+40% melee dmg) on my Scattershot Siphon X. Finally, there is the Juggernaut Shield equipment. (I like the extra shields for survivability.)

With this build, shoot the shotgun with the Disruptor ammo to stun and drop shields, and then do the headbutt thing. Rarely will you need a second headbutt. Sometimes I use this character to solo Bronze/Silver to get Combat XP. I'm not sure he's tanky enough for a Plat solo, though... If anyone can do it, you can!

There is no reason to select accuracy/headshot over sustained fire, the enemy debuff is for the entire squad and all forms of damage. The accuracy buff is for that gun.

This is another one of those "good on paper, but not in the game" situations. The only time you'd want headshot weakness are on enemies that are more than likely to die from the single headshot anyway. 2% debuff per shot is greater damage than 20% headshot weakness, unless your literally playing solo.

So I tried this build out last night and while the headbutts were very powerful (kill an Adhi with one melee), Berserkers have too much armor. Dealing with one was easy enough, but two or three at a time became unmanageable. If they headbutted me, my shields would drop and my health would take a hit. Since headbutts don't regain my health, I had to resort to my Scattershot, which lowered my DPS. This would give the Berserkers time to flak cannon or headbutt me back, and I was constantly at low health. By the time I got it back, more Berserkers would appear to start the process all over again.

I think it could work in a team Platinum, but it's tough going in a solo.