Rokkits, Power Klaws and Boyz. Lots of Boyz.
The point that the others have been making is perfectly correct. A mob of orks charging at tinboyz will take massive casualties from small arms fire. The marine player will sit back feeling all smug as he kills lots and lots of orks.
After about two turns, though, he'll start to notice that whilst he is killing lots of them, he doesn't seem to be killing that many
proportionately to the number still running at him!
Using random mathshammer is something I'm not a fan of on general principles as it doesn't reflect details that well (like cover, special weapons, etc) but it's a good way to illustrate a point.
A quick demonstration of just how hard footslogging boyz are to stop
10 marines = 25 boyz.
Starting on a marine turn when da boyz have first come into bolter range. Average of 3 kills, which can come off the back rank - 5th edition rules allow you to take a casualty on any 'identical model' in the mob, even if that model is out of range:
Now come 5th edition, the boyz will run - making 6" and an average of a further 3.5". The units are now 14.5" apart. Running means no shooting, which in turn means no marine casualties
The marines can stand and shoot, but will be a bit stupid to do so, since if the orks
Waaaggh! they have a good chance of making a 15" move and charging without the marines getting a burst of rapid fire in. More sensible is to advance 3" or so themselves and unload rapid fire into the mob, getting an average seven kills - quite impressive (small arms on light infantry will do that!).
Now, having spent two turns being the space marines' whupping boyz, the orks get to fight back. Sluggas aren't that effective but given 15 of them you'll do a bit of damage. More importantly with countercharge moves, everyone gets properly involved and you can bring numbers to bear. The assault will be bloody - with each ork boy generating as much hitting power as a striking scorpion aspect warrior! Between shooting, hacking, and No Retreat! wounds you're looking at taking down over half of the marines...
It may be the marine's turn, but once you're in combat, you're stuck there! Granted, the orks have now lost the momentum of the initial charge but at nearly 3-to-1 odds, the marines are still losing and now surrounded.....
And the next turn
You've now reached the point of the battle looking like this:
It may look heroic, but you have to admit that the space marine is screwed. On a pretty epic level.
This involved no cover, no subtlety and just charging straight through their gunfire and sucking it up.
Admittedly there were no heavy weapons or similar included, but then nor was the (cheap, easy and highly recommended) 4+ cover save from a mob of Grotz. Or Power Klaw Nob.