simple answer......look at my steam profile and notice all maps beat on all levels.
of course i got all the normal on solo, but all the orignal 5 were done on suicidal and hard as a team before level6 and we were all mostly level3s and 4s. the new suicidal and hard were mostly with 5s and 6s so it wasn't as difficult for hard, but suicidal is still pretty difficult on the new maps when you add the husk.
i owe my successes to playing with 4-5 clanmates that play together as a team. we communicate in vent and use tactics. out of a team of 6, we NEVER have more than 2 SS, but still have no trouble being successful on maps. we know who takes crossbow, who takes m14, who takes sotguns, commando, etc.... we've developed actual skill and teamwork so we don't need to rely on 6 SS to win
we know the spots to defend, we know who takes priority specimen, who takes crawlers, who welds, who's got mop up dudty....we know to leave a few to let us run to the trader to give more time to get back to our spot, we know when to suicide to spawn with free armour/weapons, save money, give money away, buy guns for others to save money.
it's just like any fps multiplayer game. playing with a real unit develops skill and teamwork that no pub team would ever be able to compete with.