A B757 LNDG AT BWI HAS A TCASII RA FROM A PASSING C210 AND TAKES EVASIVE ACTION.

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OUR B757 ACFT WAS ON L DOWNWIND FOR ILS RWY 10 AT BWI AT 4000 FT. CESSNA ACFT WAS ON CONVERGING VECTOR AT 3000 FT ASSIGNED. CESSNA ACFT WAS OBSERVED IN A CLB TCASII TA UNTIL ACFT TARGET BECAME A TCASII RA. OUR B757 HAD TO MAKE AN ABRUPT (4000 FPM) CLB TO 5400 FT MSL TO ESCAPE CESSNA. CESSNA DID NOT MAINTAIN HIS ASSIGNED ALT. SUPPLEMENTAL INFO FROM ACN 398962: RESPONDED TO TCASII RA WARNING. TOOK EVASIVE ACTION AND CLBED. (PERFORMED AS TRAINED.) SUPPLEMENTAL INFO FROM ACN 398967: WAS GIVEN 330 DEG VECTOR; DSCNT/MAINTAIN 3000 FT. UPON REACHING 3000 FT; I ENCOUNTERED HVY RAIN; VERY DARK CLOUDS (NEEDED COCKPIT LIGHTING ON); MODERATE TURB AND OCCASIONAL LIGHTNING. AFTER ATTEMPTING TO SLOW FROM CRUISE 150 KTS I ENCOUNTERED POSSIBLE MICROBURST UPDRAFT OF 1500-2000 FPM (VSI PEGGED UP). I LOWERED LNDG GEAR AND EXTENDED 10 DEGS FLAPS AND PITCHED NOSE DOWN SEVERELY TO COUNTER THE INCREASE IN ALT. CTLR QUERIED ME SHORTLY AFTER; 'CONFIRM ALT 3000 FT.' I READ BACK; 'ALT 3800 FT.' I WAS UNAWARE THAT A B757 WAS AT 4000 FT ON SW HDG. APPARENTLY HIS TCASII GAVE HIM AN RA THAT HE RESPONDED TO WITH A 4000 FPM CLB TO AVOID THE CONFLICT. ATC ADVISED ME ON FREQ OF THE NEAR MISS THEN CLRED ME FOR APCH. AT THIS TIME I EXPERIENCED A SEVERE DOWNDRAFT; APPLIED FULL PWR; SLOWLY ELIMINATED FLAPS ATTEMPTING TO MAINTAIN ALT. AT BEST; FOR THE NEXT SEVERAL MINS; I WAS ONLY ABLE TO MAINTAIN 50-100 FPM DSCNT WHILE ATTEMPTING TO MAINTAIN ALT. I FEEL THAT I FLEW THROUGH A MICROBURST; EXPERIENCING HVY UPDRAFT FOLLOWED BY SEVERE DOWNDRAFT.