Live your goal. Use the damage calculator. So if you want to say, make your Umbreon take a +1 Adamant Charizard X Flare Blitz with about 12-15% HP remaining, so after you Foul Play and kill it, you can bounce back up into stalling again, you'd go to the damage calculator and start running calcs.
Let's start with 248 HP, since an odd number reduces Sandstorm, Poison, etc. damage.
+1 252+ Atk Tough Claws Mega Charizard X Flare Blitz vs. 248 HP / 0 Def Umbreon: 397-468 (101 - 119%) -- guaranteed OHKO
Ouchies, but its workable. Workable is usually able to be seen if its barely an OHKO like that, or within the 110% as the minimum range.
So Umbreon looks like it needs a Bold Nature to live this, which is perfectly okay, since its Defense is high and worth investing into.
+1 252+ Atk Tough Claws Mega Charizard X Flare Blitz vs. 248 HP / 0+ Def Umbreon: 361-426 (91.8 - 108.3%) -- 50% chance to OHKO
Now we're cooking. Now just dump EVs into Umbreon's Defense until it hits our desired benchmark of max damage, 88%ish.
+1 252+ Atk Tough Claws Mega Charizard X Flare Blitz vs. 248 HP / 236+ Def Umbreon: 294-346 (74.8 - 88%) -- guaranteed 2HKO after Leftovers recovery
Now since we used most of our EVs, just dump the rest into Umbreon's massive SDef stat. That would be 28 SDef EVs.
So this Umbreon spread would look like 248 HP / 232 Def / 28 SDef @ a Bold Nature
If you need more help, or understanding, or want more in-depth EVing, use this guide here: http://nuggetbridge.com/articles/bulking-guide-defensive-ev-spreads/
tl;dr Make your own spreads, and look at the guide. Its efficient and takes all of 5 minutes usually.