Dawn of War II was a horrible game compared to the first. My tactic as a space marine player on DoW II is to rush the points, and continue rushing. Defense is very difficult unless you have a sizeable force, and if you do decide to engage in combat, you must outrank the other player's squads, otherwise it's a waste of manpower (in DoW II, this makes all the difference between victory and defeat). Remember to supply your power nodes with generators, and always supply the closest power node first, as it's much easier to defend this if necessary.
Try to lose as little of your forces as possible, fall back constantly if things are not going your way. Do not lose your commander, or you will lose the game (unless you're in the later stages where this is okay).
Use attack move more than the move command when exploring no-man's land. Your men will make better use of cover.
Do not underestimate transports in DoW II. The ability to reinforce away from your base is an extremely valuable trait. However, don't overestimate them either. Transports are fragile, you'll need a good force to keep them safe.
Remember, melee trumps shooting unless outnumbered. Good jumps with assault marines or stormboys can devestate enemy lines. Suppression weapons are useful if supported, so make sure you have a few, but don't spam them, because they'll just be torn apart.
All in all, it's down to luck from troop choice and upgrade choice if you follow the above recommendations. Each army has its own perks, so try them out and see which suits your style. Tyranids outnumber and prove more than a match for most armies, but when taking casualties become vulnerabilities. Eldar are very specialist and deal a lot of damage, but have fragile units. Marines are tough and can hold up in most situations, but casualties also make them vulnerabilities. Orks are really all-rounders that need a mix of troops for a strong force, but tend to get outranked by forces of the same number. Chaos use specialist tactics and are better away from generalised combat. You choose how to go about it.
But I recommend you leave this game for the original. Sure, the original is graphically lacking, but the battles are on a larger scale in the first, and more focused on base building and orients around classic, good strategy. Defense is a lot more plausible, and the tactical thinking is required, but to a different degree than DoW II. I also prefer the campaign styles in the third and forth game. (But the first and second game have a fun linear and semi-linear campaign too!)