Okay, I'm not a Dark Angels player but the obvious thing you're missing is any vehicle support. You can do without, but it'll leave enemy anti-tank weaponry (usually high strength and AP1 or AP2) freed up completely to focus down your Terminators, so if you go that route make sure you pick up some Storm Shields to tank those.
Guilliman is the other obvious target... Since you're building the list around him, that actually gives you options. While he's tough, drowning him in saves is the one thing that probably works. However, it's going to take a LOT of effort and as he needs to be focused, you might be able to get away with taking risks elsewhere, or get Guilliman into melee.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.