We present the results of combined deep Keck/NIRC2, HST/WFC3 near-infrared and Herschel far infrared observations of an extremely star forming dusty lensed galaxy identified from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS J133542.9+300401). The galaxy is gravitationally lensed by a massive WISE identified galaxy cluster at $zsim1$. The lensed galaxy is spectroscopically confirmed at $z=2.685$ from detection of $rm {CO (1 rightarrow 0)}$ by GBT and from detection of $rm {CO (3 rightarrow 2)}$ obtained with CARMA. We use the combined spectroscopic and imaging observations to construct a detailed lens model of the background dusty star-forming galaxy (DSFG) which allows us to study the source plane properties of the target. The best-fit lens model provide magnification of $mu_{rm star}=2.10pm0.11$ and $mu_{rm dust}=2.02pm0.06$ for the stellar and dust components respectively. Multi-band data yields a magnification corrected star formation rate of $1900(pm200),M_{odot}{rm yr^{-1}}$ and stellar mass of $6.8_{-2.7}^{+0.9}times10^{11},M_{odot}$ consistent with a main sequence of star formation at $zsim2.6$. The CO observations yield a molecular gas mass of $8.3(pm1.0)times10^{10},M_{odot}$, similar to the most massive star-forming galaxies, which together with the high star-formation efficiency are responsible for the intense observed star formation rates. The lensed DSFG has a very short gas depletion time scale of $sim40$ Myr. The high stellar mass and small gas fractions observed indicate that the lensed DSFG likely has already formed most of its stellar mass and could be a progenitor of the most massive elliptical galaxies found in the local Universe.
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