That. Pask is good against pretty much everything: light-infantry, heavy infantry (with rending can go after both 2+ and 3+), light-medium-heavy tanks (with rending it can glance the gak out of AV14), with this amount of shots you can even shoot down flyers without skyfire. It's onyl down side is the 24" range, so you need to get into your enemy's face. Or wait for him to come.
Some explanation: S5 Heavy 20 by itself is really good against light infantry. Pask gives it BS4, re-roll 1s on hit and wound, re-roll armor and rending. Thins means lot more of your 20 shots will hit. If more of your shots hit, you'll have more 6s on to wound and armor penetration. Also you can re-roll 1s on wound and full re-roll for penetration, that means even more rending. With rending you can wound 2+ saves (as it gives AP2 against non-vehicles), with two 6s you can even glance AV14. With the sheer number of hits you have a nice chance to shoot down flyers in a turn (you can add prescience for better effect).
Since rending is only goes for the main cannon, there is no point in addig more
HB shots, as with Heavy20 you are already good anti-infantry. So add lascannons and multi-meltas for more
AT power.