Every time you beat an EG you spawned, their avarage level goes up a little. During the last Elite Guard event, I encountered SEGs with both 1,000,000 health and SEGs with over 70,000,000 health right after eachother. 1,000,000 I can handle with a weak attack. 70,000,000 I couldn't even handle with a single full attack. I roughly categorized them in Double, Normal and Small sizes.
So, eventually, especially if you kill every single EG, you'll run into SEGs which you just can't beat without pudding, halving or a friend with much more SED than yourself, no matter how high your own SED is.
The trick that worked for me, was to have them level out just around the bracket where I could generally beat them, so where I can beat the Normal size with a full attack or 3-5 weak attacks, but needed a big hitter or halve its health for the Double Sized SEGs. Lacking the bigger hitter than myself in my friendslist, halving it had the same effect.
Scaring normal EGS meant I could move onto the next SEG without raising the avarage level any further above my capacity.
In the end, I'm generally happy if I make the top 300 rankings. Any higher than that is welcome, but not my aim.