Fighting White Scars

wow? all bikes and speeders? basically their entire point line is T5 so invest in plasma cannons with Cleanse and Purify traits so you can field 2 for each tactical squad. They are strong enough to kill everything in their army in one hit....needing only a 2 to glance off speeders also. Realize that the bikes have to get close to you so you have at least one turn to pelt them with plasma, but make sure there is a small meatshield of marines in front of the plasma cannons so when they do reach your lines your plasma cannons can keep firing to keep the speeders away.

Dreadnoughts, if kept out of the landspeeder meltagun range, can kill bikes in one hit with a power fist so maybe hide two near your lines to counter the bikers. Keep the powerfists stuck in combat with tactical squads (in the same assault) so it cannot reach the dreadnought, unless you are in direct combat with the powerfist biker, at that point just kill him....your 3-4 powerfist attacks at I4...something's gotta give.

Landspeeders are a problem because bolters tend to bounce off but plasma cannons can still work wonders but some MS will suffice.

All things rounded up.....you better outnumber the opponent at least 2:1 and have lots of plasma cannon stuff (MS krak missles....anything with AP3 strength 8). If they boost forward for an invulnerable save maybe have your own fast attack counter them because usually they are close enough. IT does depend on how many points u decide to play....small games will be difficult for biker armies.