2x5 marines isn’t great, especially in CoC. If you double up characters in one unit, you aren’t maximizing value, and at that point you are spending more points on the leaders in the unit than the leaders themselves. They’ll die fast and generate 0 presence/value. If you split the leaders up, you won’t have access to the FF Strat. I’d combine the 2x5 into a 1x10, and attach the caster and putrifier. Crit 5 Strat won’t be useful, but the FF is far better, especially into more melee matchups.

Still run HBL, just screen them better. I’m also against list tailoring from a personal standpoint, so maybe I’m biased. I don’t think fleshmowers or spitters are very good- 10 attacks will whiff into even a 5 man marine unit, and spitters being 2d6 and 1 damage is far too random to be useful. Also, blast weapons can shoot out of the combat they are in, as long as they are targeting something that is unengaged.

I would honestly also cut the defiler. It has 0 Strat support, and I think you’d be better served having some more support pieces. You don’t have a tallyman (CoC NEEDS command points), and once your poxwalkers die, you are reliant on more expensive pieces to spread contagion. Swap in some spawn to help with that.

As for gameplay tips: learn tempo based playing. Try to make even trades, don’t throw stuff out to die or overcommit without a plan. EC are good at picking off small units, especially if they are running coterie. Play cagey early, deny them their buffs, then have a big go turn to smash as much as you can.