The Mantis is pretty legit on Gold, on an infiltrator.
Just slap on the HVB and some phasic and you're good to go.
I find the SI is the best infiltrator for the Mantis on gold: energy drain means you can get away with other ammo type than phasic, which increases your damage output considerably against bigger units: disruptor rounds are great against geths (tech bursts everywhere) and cerberus (cloak, ED, headshot will kill most mooks, plus disruptor rounds buy you some time by staggering enemies), while warp rounds are great against reapers and collectors; inc. ammo is also a good choice, for the extra fire explosion damage.
That said, the Mantis on any infiltrator is totally viable on Gold 
It works well on soldier with short duration adrenaline rush build. Slightly lower damage then infiltrator with SR damage boost but AR will instantly reload it and turn it into a two shot SR.
http://kalence.drupa...104535!!!F..AGG
I would suggest a hybrid concussion shot / AR build
for example, a Mantis vanilla HSol using disruptor rounds can one-shot nemeses and centurions with a headshot + conc shot, with the advantage of a much shorter cooldown than using AR, easy-ish reload hiding, and in the case of the centurion, a tech burst. Against bigger units, going shot - conc shot (tech burst/fire explosion) - reload hiding - shot - AR - shot is a lot of damage... and conc shot is invaluable in tech teams too.
My problem with this build is that ever since I tried the double-tap Claymore, no other weapon feels right on an AR specced HSol 