Elise's strength comes from her kit. I know, thats a sucky answer but im trying to be brief. Since youre plat I'm gonna assume you can understand how all of this synergizes. She has %damage, 2 gap closers, a stun, an attack speed steroid, high base stats, and an execute.
At level 3 when most people have 3 abilities. She has 6.
Her kit allows her to gank extremely early compared to other junglers with a relatively high success rate. Why is Elise so good? Because not only does she counter Devourer junglers, who are dominate atm, she is extremely flexible build wise and can work in any team composition.
If you can gank successfully with her, at the very least you will get a summoner or back, which is huge.
As far as runeglaive vs cinderhulk goes ... I never build cinderhulk on her. I tried it for a while and disliked it, it causes her damage to fall off a cliff. I like playing her as more of an assassin than tank. But to each their own.
No one should be invading you as Elise. If they do, they have a death wish. At the moment she is probably the strongest early game jungler in the game. The only people that can reliably duel her in her jungle are pantheon, rek'sai, xin, and lee. Everyone else is an easy kill.
Depending on how you build Elise determines how you will play her in team fights. I go RG > Sorcs > X
If your team already has a lot of tankiness you can go pure glass cannon with Rylais, Liandrys, Void, Deathcap. I dont suggest this but its up to you.
If your team has no tanks you can go: RG > Sorcs > Rands/Visage, etc ...
Thats my usual build path. I play her as an assassin/bruiser hybrid. With RG, Sorcs, and Rylais she has enough damage to 100-0 any squishy in the game. After that building tanky gives her insane survivability and the ability to be your teams front line and initiation.
With this build path I play a few ways: look for picks mostly, but adjust accordingly. If the enemy squishy is constantly out of position I will stay in human form, land a stun, and initiate for my team. If not, I will look for a flank or dive their back like with rappel. In either case, you just won the team fight unless you are incredibly behind or your team is full of potatoes.
