Like they said in the comments, the base power is 120. Assuming we don't consider the effects of 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 attack would be 214 and Sylveon's defense would be 166. The damage formula is ((2level+10)attackbase power)/(250defense)+2. If we substitute all variables for their values, we get ((2100+10)214120)/(250166)+2. I'm pretty good at math, and 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 anything between 334 and 393 damage. Unfortunately, Sylveon's max hp happens to be 394, so under these conditions, it's impossible to get an OHKO. Those fairies are so annoying.