Like they said in the comments, the base power is 120. Assuming we don't consider the effects of rounding, items, status conditions, stat changes, critical hits, any moves other than heavy slam, EVs, natures, and both Pokemon are level 100 and have perfect IVs, Bronzong's physical attack would be 214 and Sylveon's physical defense would be 166. The damage formula is ((2*level+10)*attack*base power)/(250*defense)+2. If we substitute all variables for their values, we get ((2*100+10)*214*120)/(250*166)+2. I got 131. If we then factor in STAB and the type chart, we get 393. Then, there's a random multiplier between 0.85 and 1 (full details here), so we do **between 334 and 393 damage**. (though it's probably be a few points off from a properly rounded calculation) If we're still ignoring EVs, Sylveon's max HP is 331, which happens to be just enough for a guaranteed OHKO.